﻿90 BEITISH FOSSIL CEPHALOPODA. 



Carboniferous Limestone of Coalbrookdale, and it has therefore been supposed by 

 many to be a Carboniferous species, and that the Silurian form must be a different 

 one, though its identity was persisted in by Sowerby. It is to be noted that Prest- 

 wich, in his paper " On the Geology of Coalbrookdale," records the species from the 

 Wenlock Limestone and other Silurian rocks of the district, and not from the Carboni- 

 ferous. Moreover, in the index of localities appended to the ' Mineral Conchology,' it 

 is stated to come from the Wenlock Limestone. An examination of the type speci- 

 men, now in the British Museum, confirms the identity of the common Silurian 

 species with it. The section is elliptic, the ratio of the diameters being as 11 to 12. 

 The rate of increase of this portion is 1 in 11. The whole is apparently body- 

 chamber, which has therefore a length of four times its basal diameter. The aper- 

 ture is not accurately seen, but is apparently quite simple. The ornaments are : — 

 first, rounded, separate ribs, with wider, slightly concave interspaces ; these are 

 oblique about 3° on the narrower side, and so undulate on the broader, and are 

 distant ^ the diameter ; at a distance of f inch from the aperture these ribs suddenly 

 cease : secondly, a number of elevated fimbriated riblets parallel to the ribs ; 5 are 

 crowded on the summits of the ribs, and 8 occupy the interval ; they are not con- 

 spicuously grouped so as to form longitudinal bands, but their breadth is about 

 ^ the distance of the ribs ; the concavity of the festoons is very feeble. The only 

 septum seen lies in the interspace of two ribs, and is parallel to them ; the convexity 

 of the septal surface is £ the long diameter. The siphuncle is nearly -|- the diameter 

 of the shell, and lies on the shorter diameter, ^ of its length from the side towards 

 which the ribs and septa slope back ; the form of its elements is not seen. Length, 

 4 inches ; smaller diameter, 1 inch. 



General Description. — The section of this truly variable species is certainly some- 

 times circular, at others it is a little quadrate, but usually is found having unequal 

 diameters ; the extreme ratio, without obvious compression, being 11 to 13. No 

 curvature has been observed. The rate of increase is seldom so great as in the type, 

 generally varying from 1 in 10 to 1 in 14, and often becoming much less rapid in 

 the body-chamber. The latter is very seldom certainly observable ; in one it 

 reaches a length of 5^ inches and a compressed diameter of 1\ inches, without 

 showing any signs of the aperture. Of the ornaments, the transverse ribs are of 

 very varying character, but always present in the true species. Sometimes they 

 are extremely elevated and have a tendency to imbricate backwards (fig. 5) ; some- 

 times they are the feeblest possible undulations of the surface (fig. 6), but ordinarily 

 they are rounded and separate, as in the type. The interspaces are concave and 

 wider than the ribs : these are generally undulating on the broader side, but often 

 look quite direct, as seen; they vary in distance from \ to ^ the diameter when 

 uncompressed, but in flattened examples they may appear much closer. The feeble- 

 ness of the ribs, though usually met with on large examples, is not always a 



