New Zealand Recent Thyasiridae (Mollusca)
[Read before the Wellington Branch, June 10, 1948; received by the Editor, June 28, 1948.]
In Suter's Manual of New Zealand Mollusca (1913), two New Zealand Recent species of Thyasira were listed, the “cosmopolitan” Thyasira flexuosa Montagu and T. otagoensis Suter n.sp. Iredale (1930, pp. 392–394) discussed the Australian shells previously listed as flexuosa and separated two genera, Prothyasira for the reception of two species from New South Wales and South Australia, and Parathyasira for a New South Wales species. Two regional subspecies, Parathyasira resupina neozelanica Iredale, and Prothyasira peroniana peregrina Iredale, were described from New Zealand. Thyasira otagoensis Suter was not discussed by Iredale, but was later listed under Parathyasira by Powell (1946). Examination of the available neozelanic Recent material in an endeavour to identify Fiordland and Pliocene shells has led to recognition of two further generic groups in New Zealand. None of the New Zealand forms has previously been figured well enough to allow ready identification. In this paper all Recent forms now known are figured. Types and figured specimens are in the Geological Survey Collection and in the Suter Collection, now at the Geological Survey.
The writer is grateful to Mr. R. K. Dell, Conchologist, Dominion Museum, for the loan of material from the collections under his care.
Genus Thyasira Lamarek, 1818
Type (by monotypy): Tellina flexuosa Montagu. (Recent, Europe)
Comparison of Boreal specimens of the genotype with Iredale's figures of the type of Prothyasira and with specimens of Prothyasira peroniana peregrina Iredale does not show any marked conchological discrepancy, and, although, in this conservative group, anatomical differences may justify separation as observed by Iredale, I follow Cotton and Godfrey (1938, p. 210) in synonymizing Prothyasira. It is indeed possible to consider peroniana and its races as subspecies of the wide-ranging flexuosa Mont.
Figures are given of topotypes (18 fath., Stewart Island) from the Suter collection. There are others in the Dominion Museum from Port Pegasus (Bollons Collection) and from an unstated locality, and extensive series in the Geological Survey from various depths (2 to 58 fathoms) in the Southland Fiords, collected on the New Golden Hind Expedition of February, 1946. These are consistent within themselves, narrowly trigonal, deeply sulcate, and can be distinguished from the Cook Strait population.
Height: 7·4; length 6·5 mm. (topotype).
Height: 10; length 8·7 mm. (largest Fiordland specimen).
Thyasira peroniana waikanae subsp. nov. (Plate 25, fig, 3).
Distinguished from the preceding race by being relatively longer, less acutely trigonal, postero-dorsal margin less arched and less flanged, posterior sulcus shallower and not so well defined.
Height: 7·8; length 7·5, inflation 2·2 mm. (holotype, left valve).
Locality: Waikanae Beach, Wellington (Geological Survey Collection, holotype and 3 paratypes; Suter Collection, 7 paratypes).
I have not seen Recent specimens of peroniana from more northerly localities.
Genus Parathyasira Iredale, 1930
Type (by original designation): P. resupina Iredale. (Recent, New South Wales)
Parathyasira resupina neozelanica Iredale. (Plate 25, fig. 1)
Two topotype valves are figured to supplement Iredale's description: radial sculpture is largely absent, as he noted, the anterior fold is not always developed, and New Zealand shells are rounder than both the described Australian races.
Localities: 110 fathoms off Great Barrier Island (holotype, Australian Museum), and 2 figured topotypes, New Zealand Geological Survey Collection; 75 fathoms off North Cape (2 valves, Dominion Museum).
Genus Genaxinus Iredale, 1930
Type (by original designation): Thyasira albigena Hedley.
(Recent, New South Wales)
Hedley's figure of the genotype (Rec. Aust. Mus., vol. 6, pl. 661, figs. 4, 5) shows a shell similar in a general way to G. cookianus, described below, but he describes the anterior margin as sinuous, although this orientation would indicate that the umbones are opisthogyrons. Thyasira otagoensis is similar in essential features to cookiana and here considered congeneric, and if, as seems probable, the ligament is posterior, then the beaks are prosogyrous, the anterior end of the shell more produced than the posterior, and (in cookiana and albigena) the marginal sinnosity is posterior, and not anterior as Hedley described it.
Genaxinus cookianus n.sp. (Plate 25, figs. 6, 8)
Shell minute opaque, white equivalve, inequilateral, obliquely oval, produced and rounded anteriorly; beaks low, rounded; dorsal margin gently arched, anterior and ventral margins rounded, posterior margin short and sinuous, reflecting shallow posterior sulcus; hinge plate thin, flexed and thickened in front of beaks, rapidly narrowing behind and passing beneath posterior dorsal margin; muscle scars indistinct; pallial line continuous. Sculpture, weak incrementals and stronger growth folds; interior shining, with faint radials showing in the shell structure.
Height 1·08; length 1·03; inflation 0·3 (holotype, right valve).
Locality: 27 fathoms off Cape Farewell Lighthouse, in micaceous muddy silt (collected by N.Z. Govt. Steamer “Matai”), holotype and 5 paratype valves; 1 ½ miles east of Castlepoint Lighthouse, in 28
fathoms (“Matai”); specimens in the Geological Survey Collection.
Fossil occurrence: G.S. 4102, Castlecliff, Tainui Shell-bed (CU3).
Genaxinus otagoensis (Suter). (Plate 25, fig. 7)
1913. Thyasira otagoensis Suter. Man. N.Z. Moll., p. 919.
The posterior fold and sulcus in this species are so weak that they scarcely affect the marginal outline. Otherwise, in its produced anterior end and in hinge characters, this species agrees with Genaxinus and can be located here until specimens of the genotype are compared. Suter's holotype is a closed individual, so, as no figure of the interior has yet been published, one of his paratype valves is here figured.
Localities: Off Otago Heads (dredged) type; 50 fathoms off Oamaru; several Fiordland localities (15 to 58 fathoms).
Genus Maorithyas nov.
Shell of moderate size, thin, globose, endentulous, sulcate posteriorly, beaks prosogyrous, ligament opisthodetic, chiefly internal, but visible from without, bounded by a narrow escutcheon. Hinge plate thin, shallowly excavate behind, to accommodate the ligament. Lunules well-defined. Fine radial threads on interior of shell, exterior with concentric sculpture.
Type: M. marama, n.sp.
Because of its sulcation, thin shell, and resemblance in hinge characters to Genaxinus, Maorithyas is here located in the Thyasiridae. There are a number of similarities with Cavatidens Iredale, described as an endentulous Lucinid for the shells recorded as “Thyasira” globosa Forskal,” but Cavatidens has a straighter dorsal margin, more distinct muscle impressions, is only faintly sulcate, and furthermore is described by Cotton and Godfrey, 1938, p. 199, as opisthogyrate and with a wholly anterior ligament. Why Cotton and Godfrey (1938) and Iredale (1930) orient Thyasirids so as to make the sulci and ligament anterior and the beaks opisthogyrous is not clear. I have followed Dall, Suter and others in orienting these shells.
Maorithyas marama n.sp. (Plate 25, fig. 2)
1913. Thyasira flexuosa Mont. Suter, Man. N.Z. Moll., p. 919 (in part); Atlas, pl. 63, fig. 11 (not of Montagu, 1803).
Shell white, chalky without, shining within, sub-spherical, globosely-inflated, with low beaks directed forwards. A shallow but well-defined sulcus runs from beak to posterior margin and an obsolete fold and sulcus anteriorly. Dorsal margin arched, descending in front and behind, posterior margin indented at sulcus, ventral and anterior margin broadly rounded. Sculpture of irregular growth folds, locally anastomosing. Hinge plate thin and delicate, anterior part tapering backwards to behind beak, where it is overlapped by the posterior part, which widens backwards and is excavated to form a ligamental groove tapering forwards beneath the beak, bounded by a ridge-like nymph below; a crescentic narrow escutcheon on both valves extends from below the beak to the posterodorsal angle. Lunule broad, semi-circular, defined by a low fold over which growth folds
are flexed. Interior shining, with well-marked, fine, irregular radial striae, which do not erenulate the margin. Muscle scars indistinct; anterior somewhat rounded, with a small pedal sear above it, posterior rounded trigonal.
Length 18; height 17; inflation (both valves) 13·5 mm. (holotype).
Length 13·5; height 14·2; inflation (1 valve) 50 mm. (valve figured by Suter).
Localities: Fiordland; several localities, type from Suter collection, and many paratypes. The type is from 12 fathoms, Wet Jacket Arm, and valves are not uncommon in New Golden Hind dredgings from 2 to 58 fathoms. Anchorage, off mouth of Crooked Reach, Port Pegasus, in 5 fathoms. “Alert” expedition, November 23rd, 1947 (Dominion Museum and Geological Survey Collections). Also at Waikanac Beach, Wellington, North Island; several specimens, accompanying T. peroniana waikanae, n. subsp., in the Dominion Museum.
Suter had this species labelled “Thyasira flexuosa Mont.,” and the actual valve used to illustrate flexuosa (Atlas, pl. 63, fig. 11) has been identified in the series from 12 fathoms, Wet Jacket Arm. There is a related form in dredgings made by the writer at the head of Deep Inlet, Auckland Islands, which will be dealt with by Mr. A. W. B. Powell in a report on subantarctic mollusca now being prepared. Small specimens of Maorithyas are with difficulty separated from Genaxinus olagocnsis (Suter) and the affinity of otagoensis with Maorithyas may be closer than the generic separation here adopted suggests.
Cotton, B. C., and Godfrey, F. K., 1938. The Molluscs of South Australia. Pt. 1, Pelecypoda, Government Printer, Adelaide.
Hedley. C., 1907. Mollusen from Eight Hundred Fathoms, Thirty-five Milos East of Sydney. Rec. Aust. Mus., vol. 6. pt. 5, pp. 356–364, plates 66, 67.
Iredale, T., 1930. More Notes on the Marine Mollusca of New South Wales. Rec. Aust. Mus., vol. 17, no. 9, pp. 384–407.
Powell, A. W. B., 1946. The Shellfish of New Zealand (2nd ed.). Whitcombe and Tombs. Ltd.
Suter, H., 1913. Manual of New Zealand Mollusca and Atlas of Platcs (1915). Government Printer, Wellington, New Zealand.
Fig. 1—Parathyasira resupina ncozclanica Iredale. Topotypes (110 fathoms off Great Barrier Island). X 5.
Fig. 2—Maorithyas marama n.gen. et sp. Holotype. 12 fath. Wet Jacket Arm. X 2 ½.
Fig. 3—Thyasira peroniana waikanac n.subsp. Holotype. Waikanae Beach. X 4 ⅓.
Figs. 4, 5—Thyasira peroniana percgrina (Iredale). Topotypes. Port Pegasus, Stewart Island, in 18 fathoms. X 5 ¾.
Fig. 6—Genaxinus cookianus n.sp. Paratypes. G.S. Loc. 4102. Castlecliff (Upper Pliocene). X 27.
Fig. 7—Genaxinus otagoensis (Suter). Paratype. Off Otago Heads. X 17.
Fig. 8—Genaxinus cookianus n.sp. Holotype. Off Cape Farewell, in 27 fathoms. X 30. Magniflcations approximate.