﻿168 BRITISH FOSSIL CEPHALOPODA. 



growth. Some also show signs of compression by their irregular shape, and one 

 gives thus a rate of increase of 1 in 3. The mean radius of curvature in the same 

 example is 2f- inches ; but as the mean diameter of the shell is less than half that 

 of the type, the decrease of the radius of curvature is just proportional, as it should 

 be if they were of the same species. Thus the greater curvature is a proof, not of 

 difference, but of identity ; and an example, probably referred to by Salter as the 

 young of C. sonax, which shows very little curvature, for this reason, among others, 

 I refer to another species. The most characteristic feature of the species under 

 description is its direct, imbricating, strong, grouped lines of growth. In an 

 instructive example (fig. 3) showing the decrease of curvature with age, we see in 

 the younger part these groups slope considerably backwards, as they are said to do 

 in C. atramentarium, though the examples of the latter do not show this feature 

 well. The strength of these transverse lines is variable. The body-chamber in 

 some seems to contract a little towards the aperture, the edge of which, on the 

 contrary, expands. The specimen figured by Salter to show the aperture more 

 oblique in C. atramentarium, may not belong to this species ; its curvature is too 

 small. The convexity of the septa is from ^ to £ the long diameter, and they are 

 distant from -^ to -|. The sutures are only slightly concave, and are more direct in 

 the adult than in the young. The siphuncle is always internal, about T 2 ¥ the linear 

 dimensions of the septum, and oval in shape. It may be bulbous. One example 

 shows some interrupted longitudinal lines, which may be epidermids. 



Relations. — Cyrtoceras atramentarium of Salter, described at the same time as 

 this, cannot, as above seen, be satisfactorily separated from it. The name of 

 C, Forhesi has somehow become attached to this in the Museum of Practical 

 Geology, and has thence been transferred to the ' Thesaurus Siluricus ' of Dr. Bigsby. 

 That species, as figured by Barrande, has some superficial resemblance to this, but 

 is completely cut off from it by having an external siphuncle. Much nearer is the 

 Cyrtoceras conspicuum of the same author, which differs ehiefly hy its closer and 

 more sigmoid sutures. It differs from C. incequiseptum, to which an Irish specimen 

 was referred by Baily before its establishment, in having straighter sutures and a 

 uniformly convex aperture, though its dimensions are very similar. 



Distribution. — In the Bala Beds at Rhiwlas (14), at Sholeshook (6), at Cheney 

 Longville (1), at Helmgill (1) ; in the Upper Silurian, Llanfair (1) ; and in the 

 Lower Silurian, Tramore, co. Waterford (1). They are mostly in the Museum of 

 Practical Geology. The one from Cheney Longville is a pretty specimen in the 

 Woodwardian Museum, doubtfully belonging here on account of its more regular 

 ridges of growth. The matrix of most is the ordinary Bala ash, but some of those 

 from Sholeshook are in a light yellow sandstone. 



