﻿54 BRITISH FOSSIL CEPHALOPODA. 



Genus Cyrtoceras, Goldfuss. 



1832. Cyrtoceras, Goldfuss, in Dechen's translation of De la Beche's ' Geol. Man.' 



— Campulites, Deshayes, 'Encycl. Meth.' iii. [ = end ogastrica]. 

 1844. Trigonoceras, M'Coy, 'Carb. Foss. Ireland' [ = subgenus]. 



— Campyloceras „ „ ,, 



1850. Aploceras, D'Orbigny, 'Pal. strat.' [ = mediogastrica]. 



1858. Piloceras ?, Salter, ' Quart. Journ. Geol. Soc' 



1865. Cyrtocerina, Billings, 'Pal. Foss. Can.' [ = endogastrica]. 



History. — The earliest name applied to curved Orthocerata was that of Campulites, 

 but its author, Deshayes, restricted it to those with a siphuucle on the inner side ; 

 and as the greater number of forms have this organ external, the name Cyrtoceras, 

 first applied by Groldfuss, without restriction as to the position of the siphuncle, has 

 been more generally adopted for the whole group. Quenstedt considered these as 

 forming a subgenus of Lituites, defining the latter merely by its curvature. On the 

 other hand, De Koninck, in his ' Animaux Fossiles de Belgique,' gives the name a 

 wider signification than usual, by including Phragmoceras within its limits. M'Coy 

 attempted to break up the genus, by applying to the species with a triangular section 

 the name Trigonoceras, which representing a remarkable form may, perhaps, be 

 retained as a subgenus ; and to those with a circular section, the name Campyloceras, 

 which has never gained acceptance. D'Orbigny, in his ' Cours de Pale'ontologie 

 stratigraphique,' relying too much on the position of the siphuncle, applied the name 

 Aploceras to those in which it is central, ranging them with the Nautiloids, and 

 placed those with an external siphuncle among the Ammonitoids. Finally, Billings, 

 in his ' Palaeozoic Fossils of Canada,' invented the term Cyrtocerina for the species 

 with an internal siphuncle, which is at best, therefore, a synonym for Campulites. 



Description. — The transverse section is most usually elliptic, having the longer 

 axis in the plane of curvature. It is, however, sometimes transverse, and not infre- 

 quently oval, in which case, according to Barrande, the smaller end is usually 

 accompanied by the siphuncle. Species with circular sections are rare, and with 

 triangular ones still more so. The rate of increase is usually great at some period 

 of life, so that the Cyrtocerata correspond chiefly to the brevicone Orthocerata, and 

 have their total length not many times their greatest diameter. Hence a curved 

 fragment, if the curvature be slight, might be suspected to be the earlier portion of 

 an Orthoceras ; if the curvature be great, to belong to a Trochoceras. In some of the 

 species described by Barrande, there is a diminution in size towards the aperture, 

 but this does not produce an appearance of inflation of the body-chamber such as is 

 seen in Phragmoceras. The body-chamber is usually large in proportion, and is 

 invariably so in the British forms. Barrande, however, shows that the longer 



