﻿66 BEITISH FOSSIL CEPHALOPODA. 



spire, which, however, in most cases is so slight that it has to a great extent passed 

 unnoticed. This asymmetry is so unusual a feature among Cephalopods that it has 

 been used as a generic bond to unite species which would otherwise have been 

 placed in distinct genera, from the whorls in some being in, and in others out of, 

 contact. The whorls are usually few in number ; and the commencement of the 

 shell being relatively large, there is no central vacuity in the coil. The whorls 

 are relatively narrow, and the later do not conceal the earlier ones. The section 

 may be nearly circular, elliptic, subquadrate, or sublanceolate, but is very seldom, 

 if ever, transverse. The ornaments are various, but in the majority consist of 

 transverse ribs adorned by parallel riblets ; several species appear to have been 

 smooth. The body-chamber is comparatively short, but in some species the uniform 

 curvature is not continued to the aperture, but the body-chamber more or less leaves 

 the whorls. The aperture is usually simple, but the sides may be more or less 

 pressed inwards so as to give a pseudo-complex appearance to it. The septa are 

 rather approximate, and have but little transverse convexity. The siphuncle is 

 mostly simple and external, but when the septa are very close its elements appear 

 beaded. Bpidermids are not rare on the internal casts. 



The shells referable to this genus have been for the most part called Lituites, and 

 when part of the last whorl leaves the rest they certainly approach very closely to 

 that genus ; and if also the whorls are out of contact, the only difference is the want 

 of symmetry. 



Divisions. — Barrande divides the genus into endogastric and exogastric groups, 

 but there is only one Bohemian species in the former, and not a single British one. 



Range. — There are several American and Swedish Lower Silurian species of 

 Trochoceras which are called Lituites, and the genus is found in Britain in rocks of 

 the same age ; nevertheless the genus is chiefly an Upper Silurian one, abundant 

 in Bohemia, England, and the United States, while two species only have been 

 described from the Devonian in Nassau and France. 



Genus Lituites, Breyn. 



1732. Lituites, Breyn, ' Dissert. Phys. de Poly thalamus.' 

 1808. Lituites, Hortolus, Montfort, ' Conchyl. System,' vol. i. 



History. — Breyn' s definition of the genus was that its base was in a straight line, 

 but its commencement a symmetrical unconcealed spire in one plane. He did not, 

 however, distinguish between those with the whorls in contact and those in which 

 they were disconnected, but the species described was of the latter kind. Montfort, 

 reproducing the general definition of Lituites, introduced the name Hortolus for 

 those with disconnected whorls. Since that time the name Lituites has been the 

 receptacle for all coiled forms from Silurian rocks, whether provided with a straight 



