﻿BRITISH FOSSIL CEPHALOPODA. 67 



portion or not. This latter character, however, being the essential one of the 

 genus, such references are unjustifiable. Since the creation of the genus Trocho- 

 ceras, all that are at all unsymmetrical find a place ; though some, like our own 

 T. giganteum, have a short piece that is straight. With regard to the question of the 

 whorls being in or out of contact, I have not been able to find any figure repre- 

 senting a true Lituites with whorls in contact, except the rough one of Montfort, and 

 all the fragments which have been figured under the name of " imperfecti " belong, 

 to judge by the drawings, either to Trochoceras, Nautilus, or Opkidioceras. There 

 does not appear to be a single true Lituites in Bohemia, the fragment so called by 

 Barrande having no straight piece, and the whorls are in contact ; unless the species 

 called Trochoceras arietinum belong to the genus, which in fact seems a rare one. 



Description. — The early part of the shell is coiled, but the whorls are out of 

 contact, so far as known; later on, the curvature is lost or changed, and the shell 

 continues in a straight direction, but always with more or less of an irregular curve. 

 The section is not far from circular, and the rate of increase slow. The body- 

 chamber occupies a portion only of the straight piece. The ornaments are usually 

 transverse and the siphuncle central, the sutures being simple. The aperture has 

 been described as having two lappets, which bend inwards ; but these do not appear 

 to have been observed on a complete shell. The size, including the straight piece, 

 is several inches. 



Range. — The genus is confined to the Silurian rocks, there being one species in 

 Sweden, in the Lower Silurian, one in the Upper Silurian of Britain, and perhaps 

 another, which is found also in Bohemia. 



Genus Ophidioceras, Barrande. 



1865. Ophioceras, Barrande, ' Syst. Sil. de Boheme,' plates, vol. ii. 

 1867. Ophidioceras, Barrande, loc. cit., text, vol. ii. 



History. — This was established by Barrande, as a subgenus of Lituites, for the 

 species with a short " crosse " or straight piece. 



Description. — The forms figured by Barrande have a very peculiar aspect, and 

 show generic characters very distinct from the Lituites. The shell is quite 

 symmetrical, and the whorls, except at last, accurately in contact, with very little, 

 or no, central vacuity. The rate of increase is very small. The section is more or 

 less rounded, but has a flat band running along the convex side. The last portion, 

 which would make about a quarter whorl, or more, is continued in a straight line, 

 and the termination is marked by an inflation of the shell previous to its contracting, 

 so as to form a trifid aperture. The ornaments are transverse in all known 

 species, and the siphuncle subcentral ; the sutures are simple. They are all of 

 small size, not exceeding two inches in diameter. 



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