Two New Species of Tertiary Chitonidae.
[Read before the Auckland Institute, 29th March, 1928; received by Editor, 5th April, 1928; issued separately, May 30th, 1928.]
Family Cryptoconchidae Iredale. Genus Cryptoconchus Burrow.
Cryptoconchus marwicki n. sp. (fig. 1).
Intermediate valve, regularly arched from side to side, and in a lesser degree from before backwards. The central or dorsal area is a narrow longitudinal strip, rather wider at its rounded anterior end (where it projects slightly into the anterior sinus), than at its posterior end. It is sculptured with growth-lines only. The pleural areas which commence in front at about the level of the junction of the anterior and middle thirds of the central area, are sculptured with more or less ovoid flattened and faintly excavated granules which are arranged in the form of elongated triangles, each with a single granule at the apex and a row of six at the base. The remainder of the plate constitutes the articulamentum which is everywhere radially rugosely striated. It is divided on either side into a large anterior lobe and a smaller posterior lobe, the two portions being separated by a strongly marked recurved rib, beginning at the mucro and extending outwards to the incisura or marginal notch on either side; this rib presents a well marked groove along the outer half of its course. The posterior sinus is broad and straight. Valve-callus strong on the ventral surface, with a decided groove corresponding to the external rib and terminating in the solitary slit at the margin.
This specimen is the seventh plate, as compared with Cryptoconchus porosus Burrow, but differs from that species in having the jugal area broader in front, instead of being drawn out into a fine point; also in the more elaborate sculpture of the pleural areas.
Holotype in the Geological Survey Collection, Wellington; from the Upper Pliocene, Castlecliff, Wangonui.
I have named this species in honour of Dr. J. Marwick, to whom I am much indebted for his kindness in lending these specimens to me.
Family Plaxiphoridae Iredale. Genus Guildingia Pilsbry.
Guildingia tutamoensis n. sp. (fig. 2).
Intermediate valve, almost smooth, slightly polished, and broadly rounded. Jugal tract faintly and roundly carinated, and lateral areas demarcated by an ill-defined diagonal line. The whole surface
is minutely stippled, and there are two narrow parallel concentric bands of a deeper colour following the anterior and lateral contour of the plate. These are quite distinct from growth-lines, and while faintly depressed on the pleural areas, are deeply impressed on the lateral areas.
The articulamentum being altogether missing in this specimen, there are neither slits nor sinus to aid in a more decisive classification.
Holotype in the Geological Survey Collection, Wellington.
“From Muddy Creek, Tutamoe Survey District, Waipu subdivision. Just above sandstone beds at junction of large southern tributary. (Mr. M. Ongley). The beds belong to the Ihungia Series of Lower Miocene or Upper Oligocene Age” (J. Marwick).
Remarks.— The valve of Guildingia tutamoensis is so closely similar to those of the whole shell figured in Tryon's Manual of Conchology Vol. 14, Pl. 68, fig. 5 as the type of Plaxiphora biramosa Q. and G., that I mentioned the fact to Mr. Tom Iredale, who I knew had examined the holotype in the Paris Museum. In reply he states: “It shows the caelata sculpture as you describe, but it was worn and covered with corallines when alive and the scraping had minimised the sculpture, the artist neglected to show any.”