﻿226 BRITISH FOSSIL CEPHALOPODA. 



these per whorl, or they are about ^ the diameter apart. There are radial epidermids 

 on the surface about 12 to 22 per line, which pass over the ribs and continue across 

 the inside. The body-chamber is scarcely seen to become straight, and the ribs 

 scarcely die away. The septa have considerable convexity, about ^ their diameter. 

 The sutures are nearly radial, or even slope backwards, but much less so than the 

 ribs, four of which they cut. They are rather fewer than the ribs at first, but 

 become equal in number in the same space at last. The siphuncle is a little beyond 

 the centre towards the outside, about f out in an example smaller than the rest. 

 The diameter is about 5 inches, and the length of the body-chamber is about 

 the same. 



Relations. — The general structure of this shell, the shape of the section, the 

 position of the siphuncle, the character of the ribbing, are very much the same as 

 in Trochoceras giganteum, to which the specimens have been hitherto referred. But 

 in the type of the latter, as seen from the previous description, the whorls are in 

 contact. Herein lies the difference, but at the same time the ribs in the present 

 species are closer, more separate, and more continuous all round ; the aperture has 

 a different character, and the septa are closer. -They must, however, be placed in 

 the same genus, and hence, though the whorls are out of contact, that of Trochoceras 

 is adopted for the present one. The characters given by Barrande to his T. pingue 

 disagree in no respect with the figures of T. rapax, on to the smaller end of which 

 the former might very well fit. It is therefore a synonym. 



Distribution. — In the Lower Ludlow, Ledbury (3) ; and in Wenlock Shale (?), 

 Dudley (2). 



Trochoceras undosum, Sowerby, PI. XXX. figs. 5, 5a, 6. 



1839. Nautilus undosus, Sowerby in Murchison's 'Sil. Syst.' pi. 22, fig. 17, p. 642. 

 1848. Lituites undosus, Salter, 'Memoirs of Geol. Survey,' vol. ii. pi. 1, p. 352. 



Type. — Contorted. Eate of increase about 1.4, last whorl about .3 of the whole ; 

 whorls very slightly indented by the previous ones. The section is rounded quad- 

 rate, flat on the front. Thickness ^ of the whorl-breadth. The ornaments are 

 backward-curving undulations only perceptible towards the outside, where the 

 elevations rise into knots, about 11 per half whorl, which are nearly lost again on 

 the front. The body-chamber occupies \ whorl, and continues slightly beyond the 

 coiled part, the section widening out a little on the inside. The septa are concave 

 on the sides and front, coming to forward pointing angle at the edge of the latter 

 (fig. 5a). There are 22 in the last half-whorl. The siphuncle is not seen. 

 Diameter 3^ inches. From the Lower Llandovery Grits, at Blaen-y-cwm. In the 

 Museum of the Geological Society. 



