96 DEVONIAN FAUNA. 



4. Gyroceras tredecimalb, Phillips^ sp. PI. X, figs. 5 — 7. 



1841. Ctbtocekas teedecimale, Phil. Pal. Foss., p. 114-, pi. xliv, fig. 215. 

 1888. — ' — Etlieridge. Foss. Brit., vol. i, Pal., p. 167. 



Description. — Shell large, higlily ornamented, and slightly inequilateral ; 

 moderately arched, the height upon a chord of 30 mm., subtending the concave 

 side, being 5 mm. ; much flattened, the ratio of the ventro-dorsal to the trans- 

 verse diameter being 7 : 10 ; and rapidly expanding, the sides tending from each 

 other at an angle of about 30°. Siphuncle situated close to the centre of the 

 ventral margin. Chambers very concave. Surface ornamented with about 

 thirteen prominent, distant, rounded ribs, divided by flat or slightly concave 

 interspaces, of which that over the ventral or siphuncular area is much wider than 

 the rest. Surface crossed at regular intervals by very distant, transverse ridges, 

 which become tuberculous, and in the old shell even foliaceous, upon the ribs ; 

 and, between these ridges, marked, in the younger shell, with three or four distant 

 membranaceous threads, which are deeply and regularly convex towards the apex, 

 and, in the older shell, with crowded, coarse, convex striae, or growth-lines. Shell- 

 structure very thin except on the ribs. Section oval, regularly keeled or crowned 

 by the ribs. 



Apex small, recurved, with circular section, the two or three last transverse 

 ridges being reduced to mere threads, between which are seen numerous, unarched 

 and irregular growth-striae, very similar to those visible in the full-grown shell ; 

 termination hemispherical. 



Size. — Phillips's figured specimen measures %7 mm. in length, and 22 mm. in 

 ventro-dorsal and 34 mm. in lateral diameter near the mouth. 



Localities. — Wolborough and Lummaton. There are three specimens, in- 

 cluding Phillips's type, in the Museum of Practical Geology, three specimens in 

 the Battersby Collection of the Torquay Museum, and a large but poor fragment 

 in my own Collection. There is also a specimen of the apical termination in the 

 Museum of Practical Geology, and another in my Collection. 



Remarks. — This is a beautiful and easily determined species, and has been well 

 figured by Phillips from the finest specimen I know, although his figures are 

 slightly larger than the original shell, and slightly out of true proportion. One of 

 the specimens in the Torquay Museum is the top part of a much larger shell, and 

 in that the character of the ornamentation has considerably changed, the nodular 

 crossings of the ribs and ridges have become elongate membranaceous horns, 

 and the whole surface is crossed by crowded growth-lines. It is to be noticed 



