﻿124 BRITISH FOSSIL CEPHALOPODA. 



1 in 14 and 1 in 17, allowing for the flattening. The body-chamber has in one 

 example a length of 7^ inches, which is about twice the length of the basal dia- 

 meter, if that were unaltered. Perhaps the shell was thickened near the aperture, 

 which was simple and direct in outline. The surface had irregular transverse lines 

 of growth, which were sharp or not according to preservation ; they have a peculiar 

 irregularity and straightness, and every third or fourth is stronger, but without any 

 order. The septa are direct, and have in their normal state a convexity of -^ the 

 diameter, and are distant about -g- the diameter. The siphuncle is only slightly 

 eccentric on the short diameter, is f across, and has a diameter -§• of the whole. The 

 greatest diameter in a flattened shell is about 4 inches, with a length of 7| inches. 

 This description is taken from several examples, whose ornaments and association in 

 the same bed appear to indicate them to be the same species, but there are no 

 specimens which show all the characters at once. 



Relations. — The specimens called 0. irregulare by Portlock have the surface 

 ornaments more than usually well preserved, but they differ in no essential parti- 

 cular from those of the type, and there are no other distinguishing characters. His 

 type specimen of 0. complanato septum owes its peculiarity merely to the pressure 

 having been exerted on the septal surface, which has partly broken down and 

 become flat ; the eccentric siphuncle is confirmed by three other examples, and the 

 ornaments are those of the type. The specimens figured as 0. tumidum likewise 

 show similar ornaments ; but the last chamber has resisted compression, and has been 

 distorted so as to face the side. Some of the specimens, all of obscure character, 

 referred to 0. regulare, agree fairly with our present species. 



This species is very closely related to 0. politum, and it is very probable that 

 some of the specimens referred to it should belong to the latter ; nevertheless 

 there are others which from their deeply-marked lines and close septa are clearly 

 distinct from it. It is also very close to 0. mocktreense and 0. /return, from 

 the Upper Silurian, from the latter of which it differs by having its siphuncle on 

 the smaller and not on the larger diameter ; from the former it is distinguished by 

 details of ornaments, and by not having a bulbous siphuncle. 



Distribution. — All the specimens examined are from the Bala Series, Desert- 

 creat (14). M'Coy records it (as 0. tumidum) from the same beds at Tornaskea ; 

 and Phillips from the Llandeilo district. 



Orthoceras araneosum, Barrande, PI. XVII. figs. 2, 2a. 



1868. Orthocera.s araneosum, Barrande, ' Syst. Silur. de Boheme,' pi. 337-40, p. 283. 

 Query 1872. Endoceras proteiforme, Nicholson, ' Geol. Mag.' vol. ix. p. 102. 



Type. — Barrande gives a large number of figures of this species, and describes it 

 as a very variable one. The section is most often circular, but in some the ratio of 



