﻿BRITISH FOSSIL CEPHALOPODA. 65 



is uniformly, though not abundantly represented, throwing out the subgenus Aturia 

 in Eocene times, when it was pretty uniformly distributed over the temperate zones, 

 but it is now strictly confined to the Tropics, is chiefly found in the Malay Archi- 

 pelago, and affords the only living representatives of the Tetrabranchiate group. 



Genus G-yroceras, De Koninck. 



1844. Gyroceras, De Koninck, ' Terr. Carb. de Belgique.' 



1846. Spirulites, Quenstedt, 'Cephalopoda.' 



1850. Nautiloceras, D'Orbigny, ' Cours de Pal. stratigraphique.' 



History. — The name Gyroceratites appears to have been first used by Von Meyer, 

 for fossils which turned out to be Goniatites, while the true Gyrocerata were referred 

 to Spirula. De Koninck's name, however, for the evolute Nautiloids has gained 

 general acceptance. D'Orbigny alone has attempted to dismember it, restricting the 

 original name to those with an external siphuncle, and proposing Nautiloceras for 

 those with a central one. This, however, is a non-generic character, and the group 

 remains unique. 



Description. — The shell is curved in one plane, and makes at least a complete 

 whorl ; but the whorls are out of contact. The section is more or less rounded, and 

 tends to be transverse. The aperture is usually simple, but occasionally has expan- 

 sions of shell on either side. The ornaments are often complex, consisting of nodes 

 and ribs, but in some they are absent. The septa are very simple, as in an 

 ordinary Nautilus. The siphuncle may occupy any position, external, central, or 

 internal, the first position being the ordinary one. No structural marks have been 

 observed. There are no subdivisions to note. 



Range. — The Gyrocerata are confined in Britain to the Devonian rocks ; but they 

 are said to be found in the Lower Silurian of Canada, in the Upper Silurian of 

 Bohemia, and the United States, and they extend to the Carboniferous rocks in 

 Belgium. 



G-roup IV. Irregulares. 

 Genus Trochoceras, Barrande. 



1848. Trochoceras, Barrande, ' Haiding. Berichte,' iii. iv. 

 Lituttes, auctorum Anglicorum. 



History. — The genus has been so little known that since its establishment by 

 Barrande nothing has been done beyond the reference of some species to it which 

 were formerly called Lituites, and the description of some new forms. 



Description. — The essential character of the genus is the want of symmetry of its 



