﻿BEITISH FOSSIL CEPHALOPODA. 167 



are merely due to the tension of the material, cannot be determined : towards the 

 smaller end are appearances which may be interpreted as a slightly bulbous external 

 siphuncle, though this may well be doubted, all other British Lower Silurian Cyrto- 

 cerata having their siphuncle internal or unknown. 



Relations. — These two specimens are not certainly one species— but they may 

 be — and that is sufficient in these obscure forms. 



Distribution. — In the Lower Tremadoc, Llanerch (1), and in the Upper Tremadoc, 

 Garth (1). 



Cyrtoceras son ax, Salter, PI. XIX. figs. 1, la, 2, 3. 



1866. Cyktoceras sonax, Salter, ' Memoirs of the Geol. Survey,' vol. iii. pi. 25, fig. 1, p. 357. 

 Syn. 1865. Cyrtoceras in^quiseptum, Baily, ' Explanation of Sheet 167,' &c, p. 21. 

 1866. „ atramentarium, Salter, loc. cit., pi. 25, figs. 2-4, p. 358. 



1873. „ Forbesi, Bigsby, ' Thesaurus Siluricus,' p. 172. 



Not Cyrtoceras Forbesi, Barrande, ' Syst. Sil. de Boheme,' pi. 115. 



Type. — The section is elliptic in the ratio of 19 to 16^, the curvature being in 

 the plane of the long axis. The long diameter decreases at the rate of 1 in 5, as 

 measured along the convex edge. The mean radius of the external curvature is 

 6 inches. The surface has rough lines of growth, consisting of forward imbrications, 

 which are grouped in series by growing more conspicuous here and there. These 

 run nearly directly all round. The body-chamber is more compressed than the 

 septate portion. The aperture is not seen. The septa have a convexity of about 

 i the long diameter, and are at a variable distance. At a long diameter of 19^ lines 

 they are 3 lines apart, but get closer at last. The sutures are very slightly concave, 

 and nearly direct. The siphuncle is moderate in size, near the concave border; 

 curved length, 5f inches. Greatest diameter of whorl, 2f inches. From the Bala 

 Beds at Rhiwlas. In the Museum of Practical Geology. 



General Description. — The differences specially dwelt upon by Salter in separating, 

 as a distinct species from this, his C. atramentarium, which occurs in the same 

 locality, are the slight curvature of the shell and of the septa, and the greater 

 thickness of the section in the present species. These differences seem to lose their 

 importance when we consider the two forms in the relation of young and adult. 

 Only one other specimen than the type shows as great a proportionate thickness, 

 and specimens which by the directness of their septa might be thought to belong 

 to the one show the narrowness of the other. Indeed the variation is so great that 

 it is probably largely due to compression, especially as one example shows an ovoid 

 and not elliptic section. The rate of increase in the smaller form, called C. atra- 

 mentarium, is 1 in 4 instead of 1 in 5, but it is very usual for this to decrease with 



