﻿58 BRITISH FOSSIL CEPHALOPODA. 



Genus Gomphoceras, Sowerby. 



1839. Gomphoceras, Sowerby, in Murchison's 'Silurian System.' 

 1844. Bolboceras, Apioceras, F. de Waldheim, 'Bull. S. N. Moscow.' 

 1854. Stcoceras, Pictet, ' Traite de Paleontologie.' 

 1877. Mesoceras, Barrande, 'Systerne Sil. de Boheme' [ = subgenus]. 



History. — The genus was first established by Sowerby for the Silurian shells 

 with a pyriform shape and a contracted aperture, and was considered as inter- 

 mediate between Orthoceras and Phragmoceras. It has been generally accepted ; 

 the names proposed by F. de Waldheim being suggested merely by the similarity of 

 Sowerby's name to that of a genus of insects. Pictet's genus is founded on the 

 marginal position of the siphuncle — a feature which, it has been shown, cannot be 

 allowed generic importance among the Nautiloids. 



Description. — The general form is butt-shaped, the main axis being essentially 

 straight ; nevertheless one side is not always similar to the other, and when one is 

 more convex we are led towards the curvature of the Phragmoceras. The earlier parts 

 of the shell are almost unknown ; only one specimen — the type — showing a narrower 

 commencement than usual, and indicating that the apex, if ever preserved, might 

 have the form of a brevicone Orthoceras (such as that called 0. xif). The shell is 

 thick, and has very superficial ornaments, seldom preserved; but there are often 

 crenulations at the base of the body-chamber, and feebly-marked structural lines. 

 The section is never far from round, and may be transverse. The body-chamber 

 occupies nearly half the shell as commonly preserved. The aperture consists essen- 

 tially of a larger and a smaller opening, with a more or less narrow one between 

 them. The smaller, which is considered to characterise the ventral side of the 

 animal, and which corresponds to the backward sinus of the ornaments when these 

 are present, is always simple ; but the larger, in many species, is festooned by from 

 two to seven lobes, which furnish admirable specific characters. The shell is often 

 thickened round the aperture. The septa are approximate and direct, and have 

 usually very little convexity. The siphuncle is seldom marginal, but may be so ; 

 it is most usually near the centre, but towards the more convex side. It may be 

 cylindrical or inflated. The chief differences between Gomphoceras and Phragmoceras 

 are its straightness, its want of ornament, and its more commonly exogastric 

 or mediogastric siphuncle. 



Subgenus Mesoceras. — This form differs from the ordinary type in having its 

 aperture transverse instead of longitudinal, and not divided into two portions, but 

 forming a broad curved band. 



Range. — It is doubtful if the genus Gomphoceras occurs anywhere out of the 



