New Zealand Fossil Polyplacophora (Chitons).
[Communicated at the Otago Institute by Dr. H. J. Finlay, August 14th, 1928; received by Editor, December, 1928; issued separately, 29th August, 1929.]
Some two years or more ago Dr. H. J. Finlay, of Dunedin, placed in my hands a number of valves (mostly minute) of fossil Chitons, for description and determination; owing to pressure of other work it has only now been possible to study this interesting material.
There are eight species represented, two of which appear to be new and are described here-under; two belong to the Acanthochitonidae, a phylum derived from palaeozoic stock through the fossil family Protochitonidae, and three species are representatives of the Lepidopleuridae, the most primitive group of recent Chitons, through which, it is believed, the majority of living forms have been derived. Hitherto only one species of Lepidopleurus has been recorded from the Dominion, although Dr. Finlay has sent me a second species dredged in 60 fms., which is being described in a separate paper.
Subfamily Acanthochitoninae Ashby, 1925.
Notoplax (Amblyplax) oliveri Ashby, 1926.
The collection contains 4 median valves in which the shape of the valve well corresponds with this species, the sculpture also is similar except that the grains are slightly convex whereas the grains in recent examples have an almost flat surface, but I am satisfied they are conspecific with oliveri.
Locality and horizon.—Castlecliff, Wanganui (Castlecliffian, Uppermost Pliocene).
Notoplax (Amblyplax) mariae (Webster, 1908).
There is a single imperfect anterior valve of this species from Castlecliff, Wanganui (Pliocene); the insertion-plate and the form of the slits are very distinct from the anterior valve of oliveri and neither the insertion-plate nor the sculpture corresponds with foveauxensis; admittedly the granules in this example are slightly more convex than is common in mariae, but this latter is a variable species and shows more convexity in the sculpture in the juvenile stage.
The type of rubiginosus Hut. has not been disarticulated and, being a rare shell, the extent to which it is subject to variation is unknown.
Family Lepidopleuridae Pilsbry, 1892.
Lepidopleurus inquinatus Reeve, 1847.
In the collection are one tail-valve and three median valves from Castlecliff, Wanganui (Pliocene); and one tail-valve from Target
Gully, Oamaru (Awamoan, Miocene). One of these median valves has granules smaller, less flattened, less coalesced and shows bridging, but as I have noticed this latter feature between the longitudinal ribbing in some recent specimens, I do not separate this.
Lepidopleurus finlayi Ashby, 1929. (Figs. 5, 6, 7).
There are two worn portions of two median valves that I consider are referable to the new species discovered by Dr. Finlay in 60 fms. off Otago Heads, which is referred to in the introduction herein and which is being described in a separate paper.
Locality.—Castlecliff, Wanganui (Pliocene).
Lepidopleurus clifdenensis n. sp. (Figs. 8a, 8b).
In the collection are two pieces of evidently a single median valve, marked “Ischnochiton? sp.”; this valve has no insertion-plates but the sutural laminae are in an excellent state of preservation; it is certainly a typical Lepidopleurus but distinct from L. inquinatus.
Upper-side.—Colour grey, dorsal and pleural areas decorated with longitudinal rows of minute, circular, subconvex granules which, although almost touching, do not coalesce; I count about 30 of these
Explanation Of Plate.
Fig. 1.—Notoplax (Amblyplax) brookesi Ashby. Tauranga Harb. Paratype, whole shell, showing girdle spicules, Ashby Coll. × 3.5.
Fig. 2.—Notoplax (Amblyplax) brookesi Ashby. Tauranga Harb. Holotype. Anterior valve, showing sculpture and teeth. Brookes Coll. × 4.
Fig. 3.—Notoplax (Amblyplax) brookesi Ashby. Tauranga Harb. Holotype. Median valve, showing sculpture, insertion-plate and sutural laminae, Brookes Coll. × 4.
Fig. 4.—Notoplax (Amblyplax) brookesi Ashby. Tauranga Harb. Holotype. Tail valve, showing sculpture, insertion-plate and sutural laminae. Brookes Coll. × 4.
Fig. 5.—Lepidopleurus finlayi Ashby. 60 fms., Otago Heads. Holotype. Anterior valve. Finlay Coll. × 7.
Fig. 6.—Lepidopleurus finlayi Ashby. 60 fms., Otago Heads. Holotype. Median valve. Finlay Coll. × 7.
Fig. 7.—Lepidopleurus finlayi Ashby. 60 fms., Otago Heads. Holotype. Tail valve. Finlay Coll. × 7.
Fig. 8a.—Lepidopleurus clifdenensis Ashby. Fossil, Clifden, bed 60. Broken, median valve, upper-side; half. Finlay Coll. × 7.
Fig. 8b.—Lepidopleurus clifdenensis Ashby. Fossil, inside of half, same valve as 8a, showing folding-over of tegmentum. × 7.
Fig. 9.—Lepidopleurus columnarius Hedley and May. 95 fms. Auckland Islands. Median valve, showing angle of divergence. Finlay Coll. × 7.
Fig. 10.—Callochiton kapitiensis Mestayer. Kapiti Island. Paratype. Whole shell, showing longl. ribbing. Ashby Coll. × 7.
Fig. 11.—Callochiton empleurus Hutton. Auckland Islands. Median valve, showing pits and longitudinal ridges. Ashby Coll. × 7.
Fig. 12.—Callochiton klemi Ashby. Foveaux Strait. Median valve, showing pits but no ridges. Dominion Museum. × 7.
Fig. 13a.—Callochiton chattonensis Ashby. Fossil, Chatton, Gore, Oligocene. Holotype. Median valve, showing shape, upper-side. Finlay Coll. × 7.
Fig. 13b.—Callochiton chattonensis Ashby. Fossil, inside of same valve as 13a. × 7.
rows, which are regular in the dorsal and upper portion of the pleural area, but become confused towards the girdle; the lateral area is strongly raised, is only subgranulose, no regular granulose sculpture being present, but, as is so common in members of this genus, there are several irregular growth-ridges; the anterior margin of the raised lateral area shows under 20 mag. simple lens, a series of diagonal scratches or file-marks. The circular grains in the centre of the pleural area measure 60 mmm. in diameter whereas in L. inquinatus of a like size these grains are 48 mmm. or under.
Inside.—White, callus present, no insertion-plates, sutural laminae typical of members of this genus.
Locality.—Clifden (band 6A), Southland (Hutchinsonian, Lower Miocene).
Comparisons.—As compared with L. inquinatus the grains are larger and are somewhat convex, whereas in inquinatus the grains are flat-topped in the pleural and dorsal areas, also in inquinatus, in the lateral areas, the ornamentation is systematically granular and radial throughout most of the area, whereas in L. clifdenensis in this area there is no systematic, granular ornamentation at all; also it is more raised.
Family Callochitonidae Thiele, 1910.
Callochiton chattonensis n. sp. (Figs. 13a, 13b).
There were two minute median valves of a new Callochiton in the collection labelled “Ischnochiton? sp.”; one, herein described, is in a good state of preservation, the other, unfortunately now lost, was very imperfect, although the sculptural characters were sufficient to identify it.
Both valves were more strongly carinated than C. platessa, the angle of divergence being 100°; the holotype. measures 1.75 × .75 mm., the other valve 1.60 × .50 mm.
Upper side.—The dorsal area is broadly wedge-shape, smooth except for megalopores, bluntly beaked, anterior margin strikingly bowed forward. Under 62 mag. the pleural area is seen to be ornamented with about 10 rows of subgranulose, longitudinal ridges, which are separated from the adjoining row by a string of circular pits, giving to the shell, when seen under this power, a honeycomb appearance which, although shallow, is distinctive; the lateral area is raised but the honeycomb sculpture of the pleural area is absent, a few circular granules are irregularly present, the whole surface of the shell is minutely rugose; two ill-defined growth-ridges are discernible.
Inside.—White, callus strong, eaves over-hanging, spongy, insertion-plate complete on one side, teeth sharply edged, slits three; the tegmentum is folded over into the inside along the whole posterior margin of valve, the sutural laminae incomplete, shallow, the sinus between very broad and much bowed forwards.
Locality.—Chatton, near Gore, Southland (Ototaran, Oligocene).
Comparisons.—Under 62 mag. the feeble decussate sculpture of New Zealand examples of C. platessa is shown to be granulose, the granules being separated, whereas in C. chattonensis they are sub-coalesced into subgranulose ridges, also in C. platessa the longitudinal rows of circular, shallow pits, mentioned in the foregoing description, are absent; the entire absence of deep pits or coarse longitudinal ridges separates this species from the three other New Zealand forms C. empleurus Hutton, C. klemi Ashby, and C. kapitiensis Mestayer.
While one hesitates to describe a new species from such minute examples of the median valve only, the characters noted herein seem sufficiently distinctive to warrant this course being taken, especially in face of the fact that, as complete data are always wanting in the case of fossil Chitons, it is often very difficult to correlate the median valves with the end valves of purely fossil forms.
Family Ischnochitonidae Pilsbry, 1892.
Ischnochiton campbelli Filhol, 1880.
There are six examples of median valves, all minute, in various states of preservation; these I am considering conspecific with Ischnochiton campbelli, but as they all represent the extreme juvenile stage, of which I have no examples for comparison, this determination must be considered as provisional only.
Locality.—Castlecliff, Wanganui (Pliocene).
Family Chitonidae Pilsbry, 1892.
Chiton (Rhyssoplax) canaliculatus Q. and G., 1834.
There is one complete median valve and a small portion of another; the sculpture of the lateral areas is more nodulose than is the case with examples in my own collection, neither can I discern the presence of serrations on the insertion-plate teeth, which are characteristic of this genus, in all other respects the sculpture is typical of this species.
Locality.—Castlecliff, Wanganui (Pliocene).