﻿BRITISH FOSSIL CEPHALOPODA. 



Tkocboceras giganteum, Sowerby, PI. XXXI. figs. 1, 2. 



1839. Lituites giganteus, Sowerby in Murcliison's 'Silurian System,' pi. 11, fig. 4, p. 622. 



1855. Hortolus giganteus, M'Coy, 'Brit. Pal. Foss.' p. 324. 



1865. Trochoceras giganteum, Lyell, 'Elements of Geology,' p. 552. 



1873. „ „ Salter, ' Camb. and Sil. Foss.' pp. 160, 174. 



Type. — Rate of increase of coiled portion 1.5 ; last whorl .35 of the diameter. 

 The whorls are just in contact till near the straight portion ; there is very little 

 elevation to be made out. The section is subquadrate, with the umbilical edge 

 rounded, a little broader than thick (16 : 15), the straight portion becoming broader 

 in proportion. The ornaments are transverse ribs, subacute, not very separate, 

 nearly ^ the breadth of the whorl apart at first, but becoming closer in proportion 

 up to -J ; curving backwards toward the front, at first only slightly, viz. one rib 

 interval, but gradually more so, till at last they reach back six intervals ; they are 

 more or less feeble on the front, but are not quite obsolete. The body-chamber 

 begins at -| whorl before the straight portion. No change is seen towards the 

 aperture, which is not reached. The siphuncle is nearly central, but slightly 

 exterior. Its diameter on the septal surface is -j- 1 ^ the whorl-breadth. The septa 

 are direct, cutting the ribs ; they are nearly \ the whorl-breadth apart in early 

 whorls ; their convexity is £ breadth. Diameter of coiled portion 4j in., length 

 of straight portion 3f in. From the Lower Ludlow ; locality not stated. In the 

 Museum of the Geological Society. 



General Description. — The rate of increase varies between 1.48 and 1.57; the 

 breadth of the last whorl being from .31 to .35 of the whole, always bringing the 

 whorls just into contact, while the curvature remains constant. In several examples 

 otherwise agreeing with the type, decided asymmetry is observed ; in one of the 

 figured specimens (fig. 2) this is associated with the peculiar form of aperture to 

 be noticed below. The subquadrate section is characteristic, though the earlier 

 whorls are more rounded. The breadth is always greater than the thickness — 

 the proportion increasing with growth. In the other figured specimen (fig. 1), it 

 reaches a maximum of 22 : 16. The front is usually rounded, but tends to become 

 concave. The inside also when out of contact often retains some concavity. The. 

 ribs are rather of irregular character, being sometimes rounded, sometimes more 

 acute, but always rough. On the average they are about \ the whorl-breadth apart, 

 or 26 per whorl, growing closer with age. They are direct on the inside when 

 exposed, and curve back on the sides to meet at a rounded angle on the front, where 

 they either become feebler with intermediate ribs, or break up into smaller ones ; 

 the whole are generally continued to the aperture, but the ribs are sometimes 

 replaced by lines of growth. The surface is seldom well enough preserved to show 



