Sectipecten diffluxus: both forms display numerous regular primary ribs, and a disc that is folded into five or six broad radial plications. Shape and sculpture of the ears are similar. Both show the characteristic inflation of Mesopeplum, with valves slightly inflated, right sometimes more so than left, in contrast, for example, to Chlamys and its offshoots, in which the left valve tends to be more inflated than the right. However, Sectipecten diffluxus reaches a greater maximum size, and has slightly discrepant sculpture on opposite valves, right valve ribs being low and square-cut in profile, and left valves low and rounded, whereas in Mesopeplum aff. crawfordi, the ribs are high and sharp-crested, and concordant on opposite valves. Stepped growth has not been observed in Sectipecten shells. It is suggested that in post-Altonian times, probably immediately before the Waiauan, Mesopeplum stock gave rise to two distinct and divergent lineages, one of which underwent little more than a phylogenetic increase in size to culminate in the large Pliocene Mesopeplum crawfordi, while the other initially increased in size, then showed a strong trend towards amalgamation of the many narrow subequal riblets into fewer broad, strong, inflated, compound ribs, finally culminating in the highly distinctive Upper Miocene Sectipecten wollastoni and Lower Pliocene S. allani. At the same time, the main Mesopeplum stock persisted unchanged, and is still found in Recent New Zealand waters, represented by Mesopeplum convexum (Quoy & Gaimard).

Fossil and Recent Mesopeplum of the convexum type of shell seldom exceed about 55 mm in height, whereas M. crawfordi in the Pliocene at Hawera commonly grows up to about 95 mm and a specimen from the Lower Waipara (GS 7177) measures 125 mm in height; Sectipecten wollastoni commonly grows up to about 103 mm, and the largest shell collected (from Taranaki, GS 7203) measures 115 mm in height. Adults of Mesopeplum convexum, like their Tertiary forbears, have unequal ears, a ctenolium, and narrow but definite byssal notch, indicating a period of attachment at some stage during life, probably only during youth, as they are not found attached to their rocky substrate (Dell, 1954, p. 126). In contrast, adults of M. crawfordi and Sectipecten wollastoni have equal ears, no ctenolium, and completely filled-in byssal notch, indicating a much reduced period of attachment. Also it is worth recording that the distinctive “stepped” growth of Mesopeplum has never been observed in Sectipecten or large M. crawfordi, although the significance of this is not clear.

Sectipecten has not been collected from Lower Tongaporutuan rocks, but reappears as S. grangei in the Upper Tongaporutuan, in north and central Taranaki, and at Mount Bruce, Tararua, and Callaghan's Creek, Westland. Some of these specimens, particularly those from lower stratigraphic horizons, are not very different from the Waiauan S. diffluxus, and it is surprising that they are separated by the whole of Lower Tongaporutuan time.

Sectipecten grangei is succeeded by a third, very distinctive species of Sectipecten, namely S. wollastoni (Finlay). The incoming of S. wollastoni marks the advent of the Kapitean Stage by definition (Finlay & Marwick, 1947, p. 235). It should be noted that in Taranaki the incoming of the Foraminifera Bulimina aculeata d'Orbigny, characteristic chiefly of the Kapitean (Hornibrook, 1958, p. 31), precedes the incoming of Sectipecten wollastoni (see Table I).

In Taranaki, S. wollastoni is first known, from a collection made by Miss Dawn Rodley (now constituting GS 7617, N119/523), at the horizon of a shellbed (? Cave Shellbed) lying stratigraphically below the Upper and Lower Twin Shellbeds, and over a thousand feet above the Toe Toe Conglomerate containing the last known Sectipecten grangei. Several shellbeds outcrop between the Toe Toe Conglomerate and Cave Shellbed, but no Sectipecten has yet been found in them; so the exact relationship between the two species is uncertain. However, the sudden appearance of the highly specialised and remarkably invariable S. wollastoni, with no trace of transitional forms between even the latest. S. grangei and earliest

S. wollastoni, would suggest that S. wollastoni diverged from the grangei branch of Sectipecten in pre-Kapitean times, completed its evolution outside the New Zealand area or at least outside the area of which Upper Miocene records are now preserved, and invaded in Kapitean times, displacing S. grangei.

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Sectipecten grangei and S. wollastoni are also found in succession at Callaghan's Creek, Westland, where a greensand of Upper Tongaporutuan age containing S. grangei (GS 4984, 5024) is overlain by fine blue sandstone of Kapitean age containing S. wollastoni near the base (GS 3512, 5223, 153A). According to a report by H. W. Wellman and H. J. Evans (December, 1945) in the Geological Survey, the upper boundary of the greensand is sharp, but whether this represents any disconformity is uncertain. In the neighbouring Kapitea Creek there is a strong disconformity at the base of the Kapitean, but here the whole of the Tongaporutuan Stage is missing, so that the two sections are not closely comparable.

Sectipecten wollastoni is widespread in Kapitean rocks from East Cape to Southland, and is confined to the Kapitean Stage. The species is a remarkably constant and distinctive one: of more than 70 specimens seen, many of which are well-preserved complete individuals, only one specimen is significantly different. This appears to be a primitive S. wollastoni and was collected from the base of

Sectipecten diffluxus (F. W. Hutton, 1873), P1. 45, f. 1, 2, 3.

1873. Pecten diffluxa Hutton, Cat. Tert. Moll., p. 31.

1914. Pecten (Patinopecten) delicatulus Hutton; Suter, N.Z. geol. Surv. paleont. Bull. 2: 42, P1. 6, f. 4.

1928. Sectipecten diffluxus (Hutton); Marwick, Trans. N.Z. Inst. 58: 447, 454.

Holotype (TM 2753), an incomplete right valve. Figured topotype (TM. 2754), a complete individual with separate valves, from the “Bryozoan Bed”, Weka Pass (GS 6374).

Type Locality. “Weka Pass (Middle Beds)”, North Canterbury, possibly the horizon of the Hinnites Shellbed (fide Professor R. S. Allán). Age: Waiauan (Middle Miocene).

Distribution. Sectipecten diffluxus. is limited to the Waiauan of North Canterbury. Additional specimens have been collected from the Hinnites Shellbed, also from the overlying Bryozoan Bed and a somewhat higher horizon in the Glenmark Limestone Formation, Weka Pass (Wilson, in press). Only one specimen, a complete individual, has been collected outside the Weka Pass area: from Cascade Stream, Pahau River, North Canterbury, from beds of Waiauan age (Mason, 1949, p. 424).

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Redescription (based on the holotype right valve and 6 topotypes from Weka Pass, including one well-preserved complete individual, here figured). Shell moderate in size, equilateral, both valves inflated, right rather more than left. Ears large, unequal, with deep byssal notch particularly in early growth stages, becoming shallower later. Sculpture discrepant on opposite valves: ribs of right valve square-cut in profile, flat or slightly concave on top, slightly wider than their interspaces; those of the left valve high rounded and rather sharpcrested in early stages, becoming low rounded later, narrower than their interspaces. One or two right valve ribs tend to divide, either partially or completely, by the development of a central groove at varying stages in growth, a tendency not seen on the left valve. Initial number of ribs on both valves varies from 24 to 30 in 7 specimens (holotype 24). Fine close regular concentric lamellae cover the whole of disc on left valve, and interstices and sides of ribs on right valve, but are usually worn off the rib tops. Both valves folded into 5 shallow plications, more strongly marked proximally, dying out distally. Ears sculptured by 4 or 5 fine serrated riblets, often nearly obsolete on all but the right anterior ear.

Most specimens, including the holotype, have subequal, regularly-spaced ribs, but one topotype left valve has slightly unequal ribs that are somewhat grouped, with stronger ribs forming the crests of the radial plications and weaker ones between. This tendency for irregular rib development is more marked on the specimen from Cascade Stream (GS 3789), which is otherwise similar to Weka Pass shells. The Cascade Stream specimen has 24 ribs.

Dimensions. Lectotype: height, 76 mm; length, 83 mm (est.); inflation (one valve), 17 mm. The largest specimen collected (from GS 7199) measures 87 mm × 96 mm.

Sectipecten grangei n.sp., Pl. 46, f. 4, 5; Pl. 47, f. 8, 9, 10.

1927. Pecten n.sp. aff. sectus Hutton; Marwick in Morgan & Gibson, N.Z. geol. Surv Bull. 29: 41.

The species is named after Dr. L. I. Grange, Director of the Geological Survey, 1952–1956, who collected the holotype in 1925.

Holotype (TM 2755), a right valve. Figured paratype (TM 2756), a left valve. Figured specimens from Mount Bruce, Tararua (GS 3110), a right valve (TM 2757), a right valve (TM 2758), a left valve (TM. 2759).

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Type Locality. Public Works Department Quarry, Te Wera, Ngatimaru Survey District, central Taranaki (GS 1224, 7320). Age: Upper Tongaporutuan (Upper Miocene).

Distribution. Sectipecten grangei is found in the Upper Tongaporutuan at several localities in the North and South Islands, including central and north Taranaki, Mount Bruce, Tararua, and Callaghan's Creek, Westland.

Description (based on the holotype, paratype, and 20 topotypes from Te Wera Quarry, GS 1224 and 7320, all single valves). Shell small to moderate in size, equilateral, both valves slightly inflated. Anterior and posterior ears unequal, with deep byssal notch, sculptured by about 5 fine riblets, stronger on anterior than on posterior ears. On both valves, the whole disc except rib crests is crowded with fine sharp close concentric lamellae. Sculpture of right and left valves discrepant: right valve ribs flat or concave topped, slightly wider than their interspaces. Right valve ribs divided by the development of grooves on their crests and only a few intercalary riblets are produced; left valve ribs increase in number by intercalation and only 1 or 2 of the thickest primary ribs show any tendency to groove.

The new species is a difficult one to characterise concisely because it is so highly variable in rib number and arrangement. Twelve right valves have 8 to 11 major ribs (mode 10), more or less distinctly grooved from the apex. There is a range of variation from specimens such as the holotype (Pl. 46, f. 4), on which the major ribs are only superficially grooved and remain unmistakeably grouped to the ventral margin, to others on which grooving is deep from the start and the riblets soon separate, so that the initial grouping becomes virtually unrecognisable. On specimens of the latter variety each riblet could equally well be interpreted as a primary rib, but it is not strictly analogous to what has been counted as a primary rib on other specimens such as the type. Left valves have about 10 to 16 ribs at the apex, increasing to more or less double that number by intercalcation. On some specimens, for example, the paratype (Pl. 46, f. 5), ribs are very unequal, and thick ribs alternate irregularly with finer ones, the latter appearing as intercalaries at different growth stages. On other specimens the ribs are less uneven in width and nearly all could be counted as primaries, although the finer ones are mere threads near the apex. These ambiguities of interpretation on both valves preclude a consistent and accurate mode of rib counting.

Dimensions. Holotype: height, 60 mm; length, 62 mm; inflation, 11 mm. Largest topotype: height, 68 mm (est.). The largest specimen of S. grangei seen is from Callaghan's Creek (GS 4984), height, 98 mm. The topotype population consists of unusually small shells, possibly a phenotypic character, as other specimens of S. grangei arelarger and more comparable in size to preceding and following species of Sectipecten.

Diagnosis. Differs from S. wollastoni in having a far more variable, and less distinctive and regular sculpture. Right valve ribs of S. grangei are narrower than those of S. wollastoni, and more or less markedly grouped in two or threes. Ribs may be grooved once or twice but never show the multiple grooving of S. wollastoni. Left valve primary ribs of S. grangei are seldom grooved at all and never develop deep grooves at an early stage as on S. wollastoni. Differs also from S. wollastoni in having a higher average number of primary ribs (i.e., those initiated at the apex); S. grangei has 10 to about 24, S. wollastoni 7 to 10.

Differs from S. diffluxus in having a far more variable and less simple sculpture, and more markedly discrepant ribs on opposite valves: right valve ribs of S. diffluxus are hardly grouped or grooved, and left valve ribs of S. diffluxus are subequal in strength and little differentiated into primary and secondary intercalaries. Differs also from S. diffluxus in having a lower average number of primary ribs: S. grangei has 10 to about 24, S. diffluxus 24–30.

Discussion. The topotype population comprises a series of forms ranging from those with a few (about 10), broad, composite ribs divided by superficial grooves on the right valve and a corresponding number of strong primary ribs alternating

with finer intercalaries on the left valve, to those with up to about 24 riblets indistinctly grouped in two and threes on the right valve and a corresponding number of not greatly unequal riblets on the left valve. The former type of shell (having few, composite ribs) may be interpreted as advanced forms because morphologically they are similar to the following species of Sectipecten—namely, S. wollastoni, although nowhere is there a complete series of transitional forms between even the most advanced S. grangei and earliest S. wollastoni. A few S. grangei have been collected from the Toe Toe Conglomerate at Ratapiko (GS 1217, 7314), stratigraphically above the Autawa Limestone at Te Wera, but these shells are less like S. wollastoni than are the more advanced Te Wera shells.

The less advanced forms of S. grangei at Te Wera (having several, more simple ribs) are similar to an earlier population of S. grangei at Pohokura Tunnel (GS 1137), from an unnamed horizon stratigraphically below the Autawa Limestone. Six specimens at Pohokura display 20–22 ribs, average 21, which on the right valve tend to be loosely grouped in pairs or threes, and on the left valve are subequal in strength. The more advanced type of sculpture found among Te Wera shells is completely unrepresented. In rib number and arrangement, and in stratigraphic position, the Pohokura shells are intermediate between the Waiauan S. diffluxus and the uppermost Tongaporutuan S. grangei, and there may well be place here for a further systematic division. However, at present there is an insufficient number of exactly stratigraphically located specimens on which to base such a division: shells outside the immediate central Taranaki area cannot be placed in sequence within the Upper Tongaporutuan, and Sectipecten was so highly variable and rapidly evolving during this period that specimens need to be located in finer than Stage or Substage divisions if the detailed evolutionary sequence is to be described.

Three specimens from Mount Bruce, Tararua (GS 3110, mid to upper Tongaporutuan) are illustrated (Pl. 47, figs. 8–10) for comparison with the Te Wera shells, to show the variation within S. grangei. All are large shells, measuring 86 to 95 mm in height. The two right valves exhibit a polymorphism similar to that at Te Wera: one valve (Fig. 8) has several subequal primary ribs which are separate from the apex and only indistinctly grouped, whereas the other (Fig. 10) has fewer, composite ribs.

Sectipecten wollastoni H. J. Finlay, 1926), Pl. 48, f. 11, 12.

1873. Pecten secta Hutton, Cat. Tert. Moll., p. 30 (preoccupied).

1886. Pecten secta Hutton; Hector, Outline Geol. N.Z., p. 51, f. 9, No. 12.

1914. Pecten (Patinopecten) sectus Hutton; Suter, N.Z. geol. Surv. paleont. Bull. 2: 41, Pl. 9, f. 1.

1927. Pecten wollastoni Finlay, Trans. Inst. 57. 526. (nom. nov. for Pecten sectus Hutton, not of Goldfuss, 1836).

1928. Sectipecten wollastoni (Finlay); Marwick, Trans. N. Z. Inst. 58: 447, 454.

Lectotype (TM 2761), a well preserved complete individual with closed valves.

Type Locality. Callaghan's Creek, Waimea, Westland. Age: Kapitean (Upper Miocene).

Distribution. Sectipecten wollastoni is widespread in the Kapitean Stage, to which it is confined.

Redescription (based on about 70 complete and fragmentary specimens, the majority from Taranaki and Westland, a few from Marlborough, Hawkes Bay, and East Cape Peninsula). Shell large, subequilateral, both valves inflated, right sometimes slightly more than left. Ears large, subequal, byssal sinus completely obsolete in adult. Sculpture of right and left valves discrepant: primary ribs of right valve are broad, flat-topped folds, squarecut in profile, relatively wider than their interspaces; primary ribs of left valve are high-rounded and narrower than their interspaces. Right valve folds entire in early growth stages, later divided repeatedly by deep grooves, a maximum of 6 to 8 narrow sharp riblets being produced per major rib. The first groove, or pair of grooves, per rib appears at about 25

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mm to 40 mm from the apex, and further pairs of grooves appear at successive growth stages. Left valve primary ribs split by a deep median groove, which develops slightly earlier than grooves on right valve ribs; a pair of shallow grooves may develop on either side of the median groove at a later stage. On both valves the secondary ribs formed by division of the primaries remain grouped and raised above the interspaces. One to three riblets develop in each major interspace on right valve, and 2 to 5 riblets in the comparatively broader interspaces on left valve. On any one individual the sculpture of right and left valve is almost the exact antithesis, so that the broad folds with distinctive pattern of dividing grooves on right valve are matched by the wide interspaces with corresponding arrangement of intercalary riblets on left valve. Fine, sharp, crowded, concentric ridges cover the disc except for the broad rib crests of right valve where they have probably been worn off by friction with the substratum. Ears sculptured by about 5 fine riblets, strongest on anterior ear of right valve, practically obsolete on posterior ears.

Dimensions. Lectotype: height, 103 mm; length, 111 mm; inflation (two valves), 39 mm. The lectotype is the largest complete specimen collected. A shell from Taranaki (GS 7203) is estimated to have had a height of 115 mm.

Discussion. The number of primary ribs (i.e., those initiated at the umbo) is the same on both valves of a single individual, but there is a small variation among individuals from any one area and between groups from different provinces. Thus 20 specimens from Westland have an observed range of 7 to 10 ribs, mean 7.7, while 34 specimens from Taranaki have a range of 8 to 10 ribs, mean 9.2. Such slight differences are to be expected between separate populations within a species. Five specimens from East Cape, 3 from Takapau and 2 from Marlborough have means of 9.0, 9.7 and 9.5 respectively and are more like the Taranaki than the Westland population. A further difference between populations is observed in the time of onset of grooving on right valve folds: on Westland specimens the first groove appears at about 25 to 35 mm from the umbo, whereas on Taranaki specimens grooves appear at about 30 to 40 mm. On well preserved shells (for example the lectotype) it is often possible to see faint incipient grooving before a definite groove appears, matched on the opposite valve by fine intercalary riblets. Such partial suppression of grooves is seen to a greater extent on some specimens of the Chatham Island S. allani (for example, the holotype) where distinct grooves do not appear at any stage on the right valve but are represented on the left valve by quite strong intercalary riblets.

The species is distinctive and remarkably constant. There are slight variations in the number of major ribs and detailed pattern of grooving and secondary rib development, as mentioned above, but the species is nearly always unmistakable when it occurs. One specimen is notable, however, from Kapitea Creek, Westland (GS 2888). It is a fairly small shell (68 mm in height) with 7 primary ribs which begin to geminate by formation of a groove at about 20 mm from the apex on both valves. No further division occurs. Ribs of the right valve are only slightly wider than their interspaces. Compared to typical S. wollastoni, the sculpture is less elaborate, for at 68 mm from the apex on right valves of S. wollastoni the major ribs have produced anything up to 5 grooves per rib. On the Kapitea Creek shell the right valve ribs are narrower relative to their interspaces, and begin to groove at an earlier stage than on typical S. wollastoni. The specimen appears to be a primitive form of S. wollastoni, and it may be significant that it is found at the base of the Kapitean section in Kapitea Creek, whereas typical S. wollastani occur higher up the section (GS 2630, 2631).

Sectipecten allani J. Marwick, Pl. 49, f. 13, 14.

1928. Sectipecten allani Marwick, Trans. N.Z. Inst. 58: 459, f. 13, 21.

Holotype (TM 2762) a well-preserved complete individual with closed valves.

Type Locality. Momoe-a-Toa, main Chatham Island. Age: Opoitian (Pliocene). The age of the Momoe-a-Toa beds is determined by the presence of pelagic Foraminifera, primarily Globorotalia crassula Cushman and Stewart and

C. inflata (d'Orbigny) (Chatham Islands/2, F11210), indicating an age of Opoitian at the earliest (Mr. N. de B. Hornibrook, pers. comm.).

Distribution. Sectipecten allani is confined to the Opoitian Stage, and is found at the Chatham Islands.

Dimensions. Holotype: height, 97 mm; length, 103 mm; inflation, 23 mm; (largest specimen, a right valve): height, 100 mm; length, 103 mm; inflation, 16.5 mm (one valve).

Discussion. Sectipecten allani is represented in the Geological Survey collection by 12 well preserved specimens from Momoe-a-Toa, including single valves and double-valved individuals, which show a continuous range of variation in sculpture. Two right valves have 8 or 9 broad, strong, inflated, fold-like ribs, entire and smooth-surfaced from apex to ventral margin, and markedly wider than their interspaces. Interstitial riblets are almost obsolete and appear only at 65 to 70 mm from the apex. Corresponding left valves have the same number of primary ribs separated by very broad interspaces in which riblets are nearly obsolete. These forms are considered to have the most advanced and specialised type of sculpture. Four more right valves, including the holotype, have broad ribs that are lower and less convex and show faint grooving starting at about 30 mm from the apex. Another specimen has relatively narrower right valve ribs with more deeply incised grooves that start earlier. Finally, two right valves and a left valve have the least specialised sculpture. Primary ribs are low, particularly towards the ventral margin, and on the right valve are deeply split by a median groove initiated almost at the apex. On one specimen a faint groove appears later on one limb of the divided rib. On the other specimen no further grooves appear.

In discussing the possible derivation of S. allani the obvious comparison is with S. wollastoni from the Kapitean of the mainland. Both species have broad fold-like ribs on the right valve and corresponding wide interspaces and narrow ribs on the left valve. On S. wollastoni the folds are entire on the earlier part of the disc, but grooves begin at about 25 to 40 mm from the apex, whereas on the most advanced forms of S. allani the folds extend without division to the ventral margin of the shell. These forms of S. allani are closely comparable to a single right valve from the Opoitian at Kaawa Creek, which is probably a mainland development from the Kapitean S. wollastoni. Less advanced forms of S. allani (such as the holotype) are not very different from S. wollastoni, particularly when left valves are compared; right valves are superficially dissimilar because on S. wollastoni the major ribs are deeply grooved, whereas on S. allani the grooves are partially suppressed. The least advanced forms of S. allani have no counterpart in populations of S. wollastoni except perhaps for one small shell from Kapitea Creek (GS 2888) that is considered to be an early, primitive form of S. wollastoni (see above, p. 664).

Sectipecten cf. allani Marwick, Pl. 46, f. 6.

Material. A single incomplete right valve from Kaawa Creek (Opoitian Stage, Pliocene) collected by Mr. E. S. Richardson of Oruaiti School, Mangonui, Northland, who has kindly loaned the specimen to be examined and photographed.

Discussion. The shell, which measures 60 mm in height, has 10 broad, inflated, square-cut, fold-like ribs showing no signs of grooving on the surface; there are no interstitial riblets. The shell is indistinguishable from the more advanced forms of S. allani from the Chatham Islands, and differs from S. wollastoni in lacking grooves on the major ribs and lacking interstitial riblets, even at 60 mm from the apex: on the right valves of S. wollastoni grooves first appear on the major ribs at from 25 mm to 40 mm from the apex. The shell is listed as “Sectipecten cf. allani” until more specimens are found, and it can be seen whether the Kaawa Creek species is similar to the Chatham Island species in total range of variation.