﻿218 BRITISH FOSSIL CEPHALOPODA. 



var. a represents the young form and var. /3 the more adult, accounting for the 

 difference of dimensions by the distortion and compression. In the example figured 

 by Portlock (refigured PL XXVIII. fig. 5) we have the body- chamber preserved, 

 and it is seen more or less to leave the coiled portion, but the aperture is not 

 reached. The septa (fig. 6) are uniformly concave, but make a backward sinus on 

 the front, and there are 32 per whorl, but no siphuncle is seen in this specimen, but 

 one seems to show it a little within the centre. 



Relations. — Salter unites to this species the Lituites perfectus of Wahlenberg, 

 and the L. lituus of Hisinger, but the present is undoubtedly a Trochoceras. Its 

 nearest ally is the Upper Silurian T. striatum, which has a much broader whorl at 

 last, and its septa more remote. 



Distribution. — In the Bala Beds at Presteign (1), Cerrig-y-druidion (2), Trout- 

 beck (1), Sholeshook (1), Coniston (1), Desertcreat (1) ; and in the Lower Llan- 

 dovery of Llandovery (2). 



It has also been recorded by Lapworth from the Middle Silurian, Wrae ; by 

 Davies, from Bala ; and by Sedgwick, from the Coniston Limestone. 



Trochoceras asperum, Barrande, PI. XXIX. fig. 3. 

 1865. Trochoceras asperum, Barrande, 'Syst. Sil. de Boheme,' vol. ii. pi. 16, 19, p. 104. 



Type. — The Bohemian species has rather a variable rate of increase, about 1.63. 

 The last whorl is .42 the diameter, with the earlier whorls in contact. The section 

 is elliptic, the long diameter in the plane of curvature, in the ratio of 6 to 5 to the 

 short one. The ornaments are sharp oblique ribs 17 or 18 per half whorl, making 

 a deep curve on the front, where they are as strong as elsewhere, but they are very 

 feeble on the concave side. There are longitudinal as well as transverse lines cover- 

 ing the ribs, but the former are very variable. The body-chamber is continued in a 

 straight line, leaving the coiled portion ; the ribs die off towards its extremity, and 

 the aperture is simply transverse, bounded by lines of growth. The septa are as 

 remote as the average ribs, about 36 per whorl ; they have considerable convexity 

 of surface, and are only slightly curved. The siphuncle is midway between the 

 centre and the exterior. It is from the stage E, or Upper Silurian. : 



General Description. — The English specimens referred to this species have a rate 

 of increase 1.53, and the last whorl is .42 of the diameter. The whorls slightly 

 overlap, and the asymmetry is well marked. The section is an ellipse, not very 

 convex on the sides but more so on the front. The axes are in the ratio of 6 : 5, and 

 the long axis is in the plane of curvature. There are 16 very prominent subseparate 

 ribs per half whorl, curving rapidly back, and more marked on the front than on 

 the sides, and bearing parallel lines of growth. The body-chamber is partly out of 

 contact, and the ribs die away and leave only lines of growth, which by their close- 



