﻿BEITISH FOSSIL CEPHALOPODA. 125 



the axes is 9 to 8. Many show some curvature. The rate of increase is 1 in 8, in 

 the younger shells 1 in 6. The ornaments consist of a network of longitudinal and 

 transverse raised lines, of variable number, but always very close ; sometimes one set 

 and sometimes the other predominate. There are often also transverse folds of 

 growth, and occasionally irregular folds in a longitudinal direction. The septa are 

 direct, at variable distances, the average being ^ the diameter ; their convexity is 

 the same fraction. The siphuncle is sub-central. The greatest diameter figured is 

 2 \ inches. From the stage E of the Upper Silurian. 



General Description. — Two British examples of large size show a similar reticula- 

 tion ; they are both compressed ; the smallest ratio of the diameters being 4 to 3. 

 Their mean rate of increase is 1 in 7. The longitudinal lines are from 4 to 5 per 

 line, and the transverse ones are rather less conspicuous, but about equal in number. 

 The septa in the larger would appear to be more remote than in the type, being §• of 

 the mean, though less than £ the actual longer diameter ; their convexity is equal 

 to their distance, and the siphuncle is central and of moderate size, if it be really 

 seen. The greatest diameter of the compressed shell is 3 ^ inches. 



Relations. — This at first sight has the general aspect of such species as O.filosum 

 or 0. bullatum, but neither of those shows such a network of ornaments. I have 

 not been able to examine the fossil which Professor Nicholson refers to Endoceras 

 proteiforme, which appears to have been mislaid, but the description of it agrees 

 essentially with our present species as far as the ornaments, which alone are pre- 

 served, are concerned. It is scarcely justifiable to assume the presence of a 

 subgenus founded on peculiarities of the siphuncle, because of the existence of 

 ornaments which may be matched in an ordinary Orthoceras. 



Distribution. — The two specimens examined are from the Wenlock Shale of 

 Buildwas, and from the Upper Ludlow of Kendal. Prof. Nicholson's example is 

 from the Grraptolitic Mudstones of Skelgill. 



Orthoceras semipartitum, Sowerby, PI. XIV. figs. 9, 10, 11, 11a, 12. 



1838. Orthoceras semipartitum, Sowerby in Murchison's 'Silurian System,' pi. 3, fig. 9a, 



p. 904. 

 1852. „ „ M'Coy, 'Pal. Foss.'p. 316. 



1873. Tretoceras semipartitum, Salter, 'Camb. and Sil. Foss.' p. 192. 

 Syn. 1848. Orthoceras textile, Phillips, ' Mem. Geol. Surv.' vol. ii. pt. i. pi. 13, figs. 5, 6. 



Type. — The section is nearly if not quite circular; the rate of increase is 1 in 6. 

 The whole is a septate cast. The septa are direct, their convexity very moderate, 

 and their distance 3- the mean diameter. The siphuncle is nearly central, but if 

 anything on the smaller diameter, nearer the side to which it is connected. The 

 hollow left in the cast, where the shell originally was, indicates that the septal plates 



