﻿BEITISH FOSSIL CEPHALOPODA. 57 



Group II. Inflati. 

 Genus Poterioceras, M'Coy. 



1844. Poterioceras, M'Coy, ' Syn. Carb. Fobs. Ireland.' 

 1847. Oncoceras, Hall, 'Pal. New York,' vol. i. 



History. — M'Coy first gave this name to certain Carboniferous fossils which he 

 defined as " fusiform, short ; mouth contracted ; siphuncle dilated between the 

 chambers, excentric ;" and he gave a diagram, not justified by any known specimen, 

 representing the aperture as very small but simple. He refers to Gomphoceras as 

 having been given to a " fusiform species," and objects to that name, on the ground 

 of its having been adopted for a genus of insects. If that were the only objection to 

 it, the earlier names of Bolboceras and Apioceras, proposed by F. de Waldheim, on the 

 same ground would have priority ; but the only species described by M'Coy, as well 

 as his diagram, indicates a genus with the form of a Gomphoceras without its pecu- 

 liar aperture. The name has been considered by Barrande and others as a mere 

 synonym for Gomphoceras, while the species described has been relegated to Ortho- 

 ceras. Professor Hall founded the genus Oncoceras, for shells resembling the Phrag- 

 moceras in every respect, except that they have a dorsal siphuncle instead of the 

 usual ventral one ; but this is not a good generic character, and his actual type, 

 as well as others which have been referred to the genus, differ in no respect 

 from M'Coy 's genus. 



Description. — The general shape is more Or less that of a butt or vase, the earlier 

 part being not certainly known ; in some cases, however, the septal portion is very 

 like that of an Orthoceras, but an inflation commencing at or before the body-chamber 

 is the essential character of the genus. The aperture is simple and often oblique, 

 in the contrary direction to the septa. There are seldom, if ever, any ornaments ; 

 the siphuncle is usually bulbous, but neither its position nor character is essential. 

 The septa are mostly approximate. The shell commonly shows structural marks, 

 such as normal lines, &c. The genus might possibly be divided into Longicones and 

 Brevicones, like the Orthocerata, but the number of species is too small to make it 

 worth while to group them. 



Mange. — This genus, as at present defined, appears sporadically in the Lower 

 Silurian and in the Carboniferous. In the first it has been called Oncoceras, and 

 perhaps its sporadic appearance might be held to justify the separation of the two 

 groups as distinct genera. 



