﻿BEITISH FOSSIL CEPHALOPODA. 173 



Cyrtoceras corniculum, Barrande, PI. XIX. figs. 8, 8a. 



1866. Cyrtoceras corniculum, Barrande, ' Syst. Silur. de Boneme,' vol. ii. pi. 121, p. 492. 

 1873. „ „ Salter, ' Cambrian and Silurian Fossils,' p. 160. 



Not Cyrtoceras corniculum, Eichwald, 'Leth. Eossica,' pi. 47, fig. 11, p. 1288. 

 Nor „ „ Hall, ' Bep. Geol. Surv. Wisconsin,' i. figs. 1, 2, p. 41. 



Type. — Section elliptic in the ratio of 17 to 13, with the long axis in the plane 

 of curvature. The mean radius of curvature, for specimens of average size (i.e. of 

 greatest diameter 1^ inches), is 2^ inches. The rate of increase of the long diameter 

 is 1 in 3. The surface is ornamented with ridges of growth, varying in strength, 

 and distant about 2 per line, but the cast is smooth. The body-chamber is short, 

 its length being about § the long diameter of its base. The aperture is very simple, 

 having a slightly convex margin and very little sinus on the front. There may be a 

 thickening of the shell a little way below it. The septa have very little convexity, 

 and are distant ^ the mean long diameter. The sutures are only slightly concave 

 on the sides, and incline forwards towards the outside. The siphuncle is near, but 

 not at, the convex border, and is narrow and scarcely inflated between the septa. 

 Length, about 3^ inches ; greatest diameter, about 1^ inches. From stage E, or 

 Upper Silurian. 



General Description. — The specimen referred by Salter to this species, which is 

 figured (fig. 8), has an elliptic section, with the axes in the ratio of 18 to 15, the 

 curvature being in the plane of the longer diameter. Both this and others are 

 therefore rounder than the type. The mean radius of curvature is 2f inches. The 

 long diameter increases at the rate of 1 in 3, or a little more slowly in some. The 

 surface shows only fine transverse lines of growth, but the specimens are mostly casts. 

 The body-chamber is short. The aperture has a convex outline passing on the front 

 to a well-marked sinus, and below it, in one example, is a broad depression due to a 

 thickening of the shell. The surface of the septa is not very convex, and they are 

 distant ^ of the meau long diameter. The sutures have only a slight concavity, 

 and bend forwards rapidly towards the outside. The siphuncle is not satisfactorily 

 seen, and therefore probably not bulbous, but it appears to be external. The largest 

 specimen is 3 inches long by 1^ inches in greatest diameter. 



Relations. — The British species present some little differences from the Bohemian,, 

 but the reference is probably right. This has nothing to do with the species figured 

 by Eichwald nor with that figured by Hall, which have by chance been called by 

 the same name. There is very considerable resemblance between this and C. ince- 

 quiseptum in shape of section, rate of increase, radius of curvature, and distance of 

 septa, but the sutures in the present pass forwards to the outside, but in that species 



