﻿BRITISH FOSSIL CEPHALOPODA. 49 



previously named Cameroceras by Conrad, but not described ; a siphuncle of the same 

 group was afterwards named Colpoceras by Hall. The same author also founded in 

 1847 the subgenus Gonioceras for species with a fusiform transverse section. In 

 1858, Salter founded his genus Tretoceras on the specimen already figured and 

 described as Orthoceras bisiphonatum. In the ' Lethgea Rossica,' published in 1860, 

 Eicbwald gives five new names : Coch/ioceras, Trematoceras, Bictyoceras, Heloceras, 

 and Nothoceras, the first three of which had been published by him previously, the 

 first and last of which are Endocerata ; the second has deposits about its siphuncle ; 

 the third is covered by a network, possibly Polyzoan; and the fourth has a small 

 tubercle (or depression on the shell) on the normal line, but none are worthy of 

 separation. Of more recent years the tendency has been to unite rather than to 

 separate genera, except among the Ammonites, and no dismemberment of Orthoceras 

 has therefore been proposed. 



Description. — The genus Orthoceras includes all those Nautiloids which are straight 

 throughout the greater part, at least, of their length, and whose body-chamber is a 

 simple continuation of their septal portion. The form of an Orthoceras is the nearest 

 approach to a mathematical cone. The rate of increase is very variable, giving rise 

 to the two groups of Longicones and Brevicones ; it is, of course, always greater in 

 the young, the apex being invariably more or less rounded off. In some species it 

 becomes zero at last, or even negative, the body-chamber decreasing in size; never- 

 theless there is no inflation, the sides being approximately straight, and the earlier 

 part, in the latter case, being more usually lost. The body-chamber is very variable 

 in length, the more cylindrical forms naturally showing the greatest extension, but 

 giving rise to no natural subdivision of the genus. The aperture is usually similar 

 to the cross section, but mostly presents a sigmoidally curved outline on the conical 

 surface ; it is occasionally indicated by a rapid contraction or expansion, but seldom 

 by any thickening of the shell. The surface ornaments are very variable, and may 

 be used in grouping. They may be transverse ribs, or striae, or longitudinal finer 

 ribs, or a network, or all these combined ; these have no relation to the septa, 

 except that it is more common to find the latter in the interval between the ribs 

 when these are transverse. Changes with growth constantly take place in the orna- 

 ments; the most common being the loss of early transverse ribs, or the dying 

 away of the finer longitudinal ones. The internal cast does not always correspond 

 to the exterior ribs, the latter occasionally occupying the position of furrows on the 

 former, while the cast is often smooth when the exterior ornaments are fine ; the 

 actual surface, in fact, can only be certainly stated to be unornamented when fine 

 lines of growth can be detected. The shell has a tendency to split into two or 

 three layers, which are occasionally covered by structural marks easily mistaken for 

 external ornaments : some species exhibit bands of colour. The septa are usually, 

 but by no means always, parallel to the ornaments, when the latter are transverse ; 



n 



