﻿178 BRITISH FOSSIL CEPHALOPODA. 



Cyrtoceras magnum, Blake, PI. XXVII. figs. 3, 3a. 



Type. — The section is sub-sagittate, i.e. it is flattened on the convex side, but 

 more acute on the convex. The larger axis is to the shorter as 3 to 2. The 

 curvature is slight, the radius being 4^ inches when the diameter of the shell is a 

 little more than 3 inches. The rate of increase is 1 in 2. Very little body-chamber 

 and no surface is seen. The septa are slightly sigmoid, curving forward on the 

 convex side. They are distant -^ T the mean diameter. The siphuncle is unseen. 

 Length, 3^- inches. From the Lower Ludlow, Ledbury. In the collection of 

 Dr. Grindrod. 



General Description. — Whether any other examples of this species have really 

 been seen is somewhat doubtful, though specimens of some size — expanding at a 

 rapid rate, as this should do, when smaller — with a large radius of curvature are 

 seen ; but they have a more elliptic section, and some show lines of growth. The 

 septa are radial, and distant -f to -j^ the diameter, according to size, being more 

 remote at first. Their convexity is slight. The siphuncle is seen in these to be 

 large, bulbous, and external. 



Relations. — From its shape it would be quite possible that this should be part 

 of a Phragmoceras. But from its external siphuncle, the only described form it 

 could be is P. devonicans, and that has a less rapid increase. Nevertheless, were 

 this proved to have a contracted aperture, there would be comparatively little to 

 separate them. The nearest British form is Cyrtoceras subarcuatum; but if the other 

 specimens are rightly referred to the same, that species has a central siphuncle. 



Distribution. — In the Upper Silurian of Pen-y-lan, Cardiff, in beds which are 

 considered to belong to the Wenlock, but whose fauna is mixed (1) ; in the Lower 

 Ludlow, Ledbury (2) ; in the Upper Ludlow, Woolhope (1). 



Cyrtoceras fortiusculum, Barrande, PI. XIII. fig. 3. 



1866. Cyrtoceras fortiusculum, Barrande, 'Syst. Silur. de Boheme,' vol. ii. pi. 207, figs. 13-16, p. 630. 



Type. — Section oval, with the diameters in the ratio of 10 to 9, and the curva- 

 ture in the plane of the long diameter. • The narrower end is on the convex side. 

 The mean radius of curvature is 14 lines when the mean long diameter is 8^ lines. 

 The long diameter increases at the rate of 1 in 3f . The surface has only fine lines of 

 growth. The body-chamber is about as long as its basal diameter. The aperture is 

 perfectly simple, and formed by a scarcely convex curve perpendicular to the general 

 direction of the shell. The septa have a convexity of \ the long diameter, and they 

 are very close, viz. T * ¥ the long diameter. The sutures are but slightly concave, and 



