﻿46 BRITISH FOSSIL CEPHALOPODA. 



Conici. Diverging from this, and more restricted in time, is a remarkable group, 

 which, with a generally straight or somewhat curved axis, as in the former series, 

 has so strange an appearance as at once to be separable. This appearance depends 

 not so much upon the shape of the aperture, which is very various, as upon the 

 general inflation of the shell, so that, as a whole, it is fusiform, pyriform, or flask- 

 shaped. This inflation is accompanied usually by some other peculiarities of the body- 

 chamber. Either the aperture is contracted, or distorted septa are formed in it, or 

 both are combined. Moreover, the septal surfaces are commonly flat, and the earlier 

 septa often break off naturally. This group contains the genera Phragmoceras, 

 Gomphoceras, Poterioceras, and Ascoceras, and may be characterised as the Ikflati. 

 The other two groups of Nautiloids run in some sense parallel to these, but have so 

 great a curvature that they form what are known as whorls, which may or may not 

 be, but generally are, in contact. This arrangement has permanently succeeded the 

 former, or conical, as it is represented in the living Nautilus, and, when united with 

 other modifications, produced the dominant family of the Ammonitoidea. Although 

 the great curvature does not seem to be associated with any other general pecu- 

 liarities of consequence, yet this fact of its superseding the small curvature is of 

 itself sufficient to justify us in regarding it as of sufficient importance to characterise 

 a group, which, containing the genera Nautilus, Gyroceras, Trocholites, Clymenia, 

 Nothoceras, and others, may be called the Spirales. Of the above genera Gyroceras 

 is the intermediate form, leading to the most curved of the Cyrtocerata, but more 

 naturally grouped with the Nautili. The Trocholites and Clymenia form a remark- 

 able offshoot, which have been even thought worthy of separation from the whole 

 family of the Nautiloids. They are not, however, more remarkable than the Tretoceras, 

 Endoceras, or Ascoceras, among the Conici and Injiati, and the modification is the 

 same ; namely, a deep depression on the marginal surface of the septa. Finally, 

 separated from the normal Spirales by some peculiarity of form, either a want of 

 symmetry, or a loss of curvature producing a straight body-chamber, or great 

 changes of curvature, are a number of genera of peculiar aspect, which may be con- 

 veniently associated together as Irregularis. Such irregularities of form appear to 

 have been attempted at various epochs ; just as we find in later times the Turrilites, 

 Helicoceras, and Hamites, among the Ammonitoidea, but they were never very suc- 

 cessful, and one is at times in doubt how far they are worth distinguishing from 

 the normal forms. Whilst amongst the Conici the irregularity took the character of 

 inflation, so commonly as to throw other variations into the shade, in these Spirales 

 there is no preponderance of one form of irregularity over another, though the most 

 common is a want of symmetry. The genera composing this group are the 

 Trochoceras, Lituites, Ophidioceras, and Cryptoceras, We thus have the following 

 table of the genera of Nautiloidea. 



