﻿174 BRITISH FOSSIL CEPHALOPODA. 



towards the inside, and this difference may possibly be associated with a different 

 position of siphuncle. 



Distribution. — In the Wenlock Shale, Dinas Bran (1) and Duke's Quarry (1) ; 

 in the Wenlock Limestone, Dudley (1). 



Cyrtoceras isca, Blake, PI. XIX. figs. 6, 7. 



Type. — The specimen (fig. 7) is flattened, and does not give its true shape. The 

 curvature is moderate, the mean radius being 2 inches for the external curve. The 

 long diameter, which is in the plane of curvature, increases in the flattened shell at 

 the rate of 1 in 4. The surface shows feeble signs of forward imbrications (better 

 seen in other examples), about 2 per line. The body-chamber is seen for a length of 

 7 lines, and there is no change on approaching the further end. The septa are 

 very numerous, from -^ to -^ the long diameter apart, the last few being the closest. 

 The sutures are sigmoidal, rapidly curving forward in the middle of the side, and 

 bending round so as to pass straight over the convex side. The siphuncle is 

 external and bulbous. Length, 1 \ inches ; diameter, 1 inch. From the Wenlock 

 Limestone, Garcoed, near Usk. In the Museum of Practical Geology. 



General Description. — The specimen above described is not in some respects the 

 best preserved, but is taken as the type as best showing the septal characters. The 

 section is elliptic, the diameters in the ratio of 24 to 19, the longer one in the plane 

 of curvature. The curvature is never great, the type having a maximum. The rate 

 of increase is rather greater than 1 in 4. The ornaments are direct, forward imbri- 

 cations, from § to \ a line apart (fig. 6), convex towards the aperture. These cease 

 on the body-chamber, and give place to lines of growth. There is some appearance 

 in one specimen of the aperture having been contracted like a Phragmoceras, but 

 there is no other change in the shape of the body-chamber. The septa are very 

 slightly convex, very close and sigmoid. The siphuncle in all is external and bulbous. 



Relations. — This species runs very close to two others, but could not well be 

 placed with either of them. The closeness of the septa, the position and form of the 

 siphuncle, and the general rate of increase bring it very near to C. fortiusculum, 

 but that is a smooth species, while this is ornamented. It has also its sutures 

 sigmoidal, and perhaps even closer, and its curvature is less. On the other hand, 

 C. quasi-rectum, var. contraria, has ornaments of rather a different character, 

 which alone would be of comparatively little consequence, but the general shape 

 of the one is altogether different from that of the other, and the septa have a 

 greater average closeness. 



Distribution. — In the Wenlock Limestone of Garcoed, Usk (3) ; all these examples 

 are in the Museum of Practical Geology. 



