﻿STRAPAROLLUS. 



317 



249. Discohelix spinosus, Wright, MS. Plate XXV, fig. 8. 



1859. Straparollus spinosus (name only), Wright. Quart. Journ. G-eol. Soc, 



vol. xvi, p. 36. 



1873. ? — sp., Tawney. Dundry Gasteropoda, p. 36 (28). 



Cf. also — sub^qualis, d'Orbigny. Terr. Jur., vol. ii, p. 311, pi. 322, 



figs. 8—11. 



Description : 



Diameter (fair size) . . . .24 mm. 



Height . . . . .8 mm. 



Diameter of largest specimen known . . 4G mm. 



Shell depressed, discoidal, upper and under surfaces nearly equally excavated. 

 The whorls are subquadrangular, with sides almost perfectly flat, and the 

 marginal keels are largely tuberculated at intervals. The spiral ornamentation is 

 so fine as in many cases not to be detected ; this is decussated with somewhat 

 rugose strias of growth, perpendicular or curved. In the larger shells the 

 marginal tuberculation extends in ill-defined swellings or costse right across the 

 body-whorl. Aperture quadrangular, spiral and axial diameters nearly equal. 



Relations and Distribution. — This species is by far the largest of any described 

 under Discohelix- Straparollus from the Inferior Oolite of this country. 1 It most 

 resembles Straparollus s ul>sequal is, d'Orbigny, from the Inferior Oolite of Fontenay 

 and Niort, both as to habit of growth and general character, but the marginal 

 tuberculations present a considerable difference. It is also related to Straparollus 

 tuberculosus, Thorent, but is truly biconcave and dextral. From all other British 

 species of Discohelix and Straparollus, except the one next described, it may be 

 separated by the fineness of the spiral ornamentation. In many specimens this 

 is so fine that the shells appear smooth, though this appearance, no doubt, is 

 partly due to wear. 



Discohelix spinosus occurs principally in the coucavus-bed of Bradford Abbas, 

 where it is rather rare. 



250. Straparollus " tuberculosus-dexter." Plate XXVI, fig. 1. 



1854. Euomphalus tuberculosus, Thorent, sp. Morris, Cat., p. 248. 

 Cf. — — — Mem. Soc. Geol. France (1839), vol. 



iii, pt. 2, p. 259, pi. ssii, fig. 8. 



Bibliography, Sfc. — To a certain extent the specific name " tuberculosus " is 

 applicable to all species of Diseoif! i.r and Straparollus described in this Mono- 



1 Owing to unequal enlargement, and to the selection of medium-sized specimens, this fact is not 

 made sufficiently evident in the plates. 



