﻿BRITISH FOSSIL CEPHALOPODA. 29 



they are seldom great in any Nautiloid, and in some species are reduced to zero, so 

 that the outline of the aperture becomes a plane curve. 



From the simple form the change is very slight to the least complex of the 

 contracted. The slight bending in of these side lobes will narrow the centre and 

 cause the aperture to be roughly divided into two, with a broad passage between. 

 This is essentially the character of all, except a few, of the contracted forms, how- 

 ever modified. This bending in may take place without any other difference of 

 importance being noticed, as in Trochoceras and Lituites, and it may be very small in 

 amount. It is also very probable that it only occurs in the adult, or even old age, 

 as it is seen only in large specimens of the species. Another simple modification is 

 a bending in on the dorsal side to a greater or less extent, as in the species called 

 Hercoceras mirum, and in some others from the Carboniferous rocks of England. 

 When this is united to a bending in of the lobes in a small group called Opliidio- 

 ceras, a swelling of the body-chamber near the aperture, and other peculiarities, it 

 assumes generic importance. 



The apertures of Phragmoceras and Gomphoceras are still further modifications ; 

 but, from the extreme variety we meet with, it is obvious that it is brought about by 

 the general contraction of the body-chamber, to which the aperture has to accom- 

 modate itself. These apertures consist of three portions : first, the large opening on 

 the dorsal side, which may itself be unlobed or lobed in various degrees ; secondly, 

 of a smaller one on the ventral side, and usually on the slope ; and thirdly, of a 

 narrower passage between the two. The small aperture is assumed to be on the 

 ventral or funnel side, because of its constancy and of its forming a deeper sinus by 

 its position. 



6. The Ornaments. 



These, when correctly observed, with the changes that take place in them as we 

 pass from the young to the adult, form the most valuable of specific characters in the 

 Nautiloidea. The various kinds are so uniformly distributed as to yield no tempta- 

 tion to use them for greater purposes, though we may conveniently group the 

 species by them for the sake of reference. It is only necessary to define their 

 terminology. They consist usually of ribs, which either run longitudinally, from the 

 apex towards the aperture, or transversely, parallel to the section. These may be 

 either acute or rounded ; they may be called separate, if they are narrower than the 

 interspaces, and the transverse ones are undulating if they do not form a plane curve. 

 The minor ornaments are either finer ribs which stand out from the surface, and may 

 be called riblets, or impressed lines which alone should be called striae. When these 

 become irregular, they are taken to be lines of growth. If any of the tranverse 

 ornaments have in section a more rapid slope on the side nearest the aperture, they 

 are said to imbricate upwards ; if the contrary, downwards. In some cases the 



