﻿BRITISH FOSSIL CEPHALOPODA. 59 



Upper Silurian rocks, none of those recorded from elsewhere having a complicated 

 aperture preserved, and several belonging certainly to other genera. 



Genus Phragmoceras, Sowerby. 

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



History. — This genus was established by Sowerby at the same time as Gompho- 

 ceras, and he states that the name was suggested to him by Broderip, and this 

 accounts for the genus being quoted as the creation of the latter, who was neither 

 its describer nor author. Another name applied to some of its species is Campulites, 

 which M. Deshayes gave to endogastric forms of all kinds, whether of Cyrtocerata or 

 Phragmocerata, and which may therefore be a synonym of either. 



Description. — The curvature is always well marked, but is never very great ; 

 the section usually has its longer diameter in the plane of curvature. The body- 

 chamber is less inflated than in Gomphoceras, especially on the concave side ; the 

 aperture is on the same type, — namely, two larger openings connected by a narrower 

 passage, the larger one often lobated, the lobes being of an even number in all the 

 known species. There are very often feeble transverse ribs, which undulate sig- 

 moidally across the shell. The septa are usually approximate, and have very little 

 convexity ; the earlier ones do not so constantly fall off as in Gomphoceras. The 

 siphuncle in the great majority of species is internal, hence these might be called 

 endogastric ; there are, however, a few both of mediogastric and exogastric species, 

 but these are not worth separating, either as divisions or as subgenera : the 

 elements of the siphuncle are usually, but not universally, nummuloid. There are, 

 in some species, crenulations at the base of the body-chamber. 



Divisions. — Besides the general subdivision relating to the position of the 

 siphuncle, it is to be noted that Barrande makes groups of the species according to 

 the number of lobes in the larger opening of the aperture — calling them Dimeres, 

 Tetrameres, &c. 



Range. — The genus Phragmoceras has three representatives in the Lower Silurian 

 rocks of England, Bohemia, and Canada, but it is otherwise confined to the Upper 

 Silurian. The Carboniferous species referred to it have been founded on errors. It 

 is chiefly found in England and Bohemia, and there are a few in Germany, America, 

 and Canada. 



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