﻿198 DEVONIAN FAUNA. 



2. Genus. — Platyostoma, Conrad, 1842. 



These shells are low, subglobose, and umbilicated. The spire is of few whorls, 

 and increases rapidly and somewhat asymmetrically. The mouth is more or less 

 pentangular in shape. The genus occurs in the Silurian, Devonian, and Carboni- 

 ferous formations. 



Some of the species described below show considerable likeness to the 

 lanthinidse in the shape of the mouth, but, on the other hand, they appear in 

 some other respects to be related to the Gapulidx ; and, after a comparison 

 with several foreign species of Platyostoma, it has seemed best to group them 

 under that genus, and to follow Zittel in ranking them among the Velutinidse. 



1. Platyostoma sigmoidale, Phillips sp. ? PI. XIX, figs. 9, 10, 10 a. 



? 1840. Aceoculia sigmoidalis, Phillips. Pal. Foss., p. 94, pi. xxxvi, fig. 170. 

 1854. Capulus sigmoidalis, Morris. Cat. Brit. Foss., p. 239. 

 1858. — naticoides, Giebel (not F. A. Romer). Sil. Faun. Unterharz., 



p. 2(3, pi. iii, fig. 7. 

 Natica geegaeia, Barrande, MS. 

 1878. Platyostoma naticoides, Kayser. Abhandl. G-eol. Specialk. Preuss., 



Band ii, pt. 4, p. 100, pi. xvi, 

 figs. 4, 4 a. 

 1888. Aceoculia sigmoidalis, Etheridge. Foss. Brit., vol. i, Pal., p. 163. 



Description. — Shell small, spiral, very oblique, and flatfish. Spire depressed 

 and very obtuse, formed of about three flat and very rapidly increasing whorls. 

 Suture well defined, obtuse. Whorls rising convexly from the suture, and then 

 spreading out with a gradually increasing curvature till they turn with a slight 

 blunt angle to the lower parts of the shell, along which a slight shallow, indistinct 

 groove or depression is developed near the mouth. Surface ornamented by very 

 numerous, regularly undulating, longitudinal, microscopic, sharpish lines, sepa- 

 rated by shallow furrows, which are visible also on the mould, and are arranged in 

 groups of two or three, occasionally divaricating, and sometimes crossed and 

 partially reticulated by still finer spiral striae. Mouth very large, oblique, with 

 wavy lips. Shell-structure very thin. Colour black. 



Size. — Height 8 mm., width 15 mm. 



Locality. — Lummaton. There is a specimen in the Torquay Museum upon a 

 slab with Terebratula juvenis, as well as a second specimen in the same Museum, 

 and a third in my Collection. 



