﻿BEITISH FOSSIL CEPHALOPODA. 53 



Nautilus is an organ occupying a definite relation to its internal organisation, and to 

 mistake a backward prolongation of the body for a second one is simply to confound 

 superficial resemblances with actual homologies. Salter compared his genus to a 

 Gonioceras, in which the angular portion had been bent round so as to enclose a 

 tube ; but in this case we should have found the tube continuous throughout, and 

 not ceasing in the body-chamber as it actually does. Neither of the other species 

 referred to this genus 1 appear to me to have any relation to it, but to present other 

 peculiarities of quite distinct character, and serving only to show that such 

 peculiarities are of subgeneric value only. Its true relations, other than to the 

 typical Orthoceras, appear to be to Endoceras and Ascoceras : for if the backward 

 prolongation of the body had included the position of the siphuncle, it would have 

 been difficult to distinguish it from the former ; and if it had been of greater size, so 

 as to affect the general shape of the septa and produce distortion, we should have a 

 form representing among the Conici the latter genus among the Inflati. 



4. Coistoceras. — The septa bend forward in the neighbourhood of the siphuncle, 

 which is lateral, and meet at an angle forming a chevron. This is exactly the 

 contrary direction to what is usual for the neck in Orthocerata, and it is doubtful 

 how far the part over the siphuncle really represents the neck, as it is angular and 

 reaches forward three or four septal spaces. In any case the structure is so peculiar 

 as to require at least a subgeneric distinction. The other character given by 

 Barrande to Bathmoceras, which is otherwise a synonym of this, — namely, that the 

 last two or three septa are usually found incomplete, — is of no value, since many 

 Orthocerata of ordinary character, Phragmocerata, and even Ammonites, show similar 

 incomplete septa, dependent probably on accidents of preservation. 



5. GtOntoceras. — The section is flattened and fusiform, instead of making any 

 approach to a circle or ellipse. This appearance may be due to preservation ; but, 

 in the absence of any proof of this, the name must be allowed to stand. 



Range. — The genus Orthoceras has its earliest representatives in the Upper 

 Tremadoc rocks, and attained its maximum in the Upper Silurian. There were 

 many representatives in Devonian and Carboniferous rocks in every fossiliferous 

 locality. The Permian species are reduced to three, two of which are American, 

 and the other from Thuringia ; while a greater number reappeared in the 

 S. Cassian and Hallstadt beds, and others of the same age have been identified in 

 the Himalayas and California. 



1 T. semipartitum, Salter, and T. parvulum, Barrande ; the latter, as the author indicates, belongs to 

 the so-called Bactrites. 



