﻿336 



INFERIOR OOLITE AMMONITES. 



Sonn. spinifera is nearest to Bonn, cymatera, and is the morphological equiva- 

 lent thereof. It may be distinguished, however, by a more pronounced spinous 

 stage and a decidedly less-marked costate stage, by the ribs being more reclined, 

 and failing on the outer third of the whorl, the whorls being rather more gibbous 

 and being without any inner margin. The spines taking up more of the umbilicus, 

 and the much smaller size of the ribs, are the features most noticeable in this 

 form compared with cymatera. 



This species is certainly not the young of ptycta ; it is more slowly coiled, has 

 very much smaller spines which yield to the costate stage very much earlier ; its 

 costas are less strongly marked, and it is evidently more compressed. 



It is not a young form of any species of either the dominans- or submarginata- 

 stocks because of its strongly-reclining ribs ; they give a noticeable twisted 

 appearance to the shell. 



The young example figured in Plate L, figs. 14, 15, is a well-preserved fossil 

 with some of its spines complete, and it is a form by no means unknown in the 

 Concavum-zone of Bradford Abbas. Its exact agreement with the inner whorls 

 of the specimen shown in Plate LXXIV, figs. 4 — 6, proves that the latter is an 

 older example of the same species. This specimen, so far as my evidence goes at 

 present, is presumably an adult form. 



The costata-stock . 



Properly speaking, the series only consists of two common Bradford- Abbas 

 species, namely — 



Costata, the costate stage throughout life, the spinous stage very rudimen- 

 tary. 



Parvicostata, the spinous stage extremely rudimentary, the costate stage 

 confined to the inner whorls, the outer whorls smooth except for some obscure, 

 irregularly-placed, undulate costae. 



These two forms illustrate two phases of retrogression ; and parvicostata looks 

 like a terminal of a series. There is some variability among the specimens of the 

 two forms, and they shade one into the other. 



Their descent is a matter of considerable uncertainty. At first glance, costata, 

 (.■.specially the example figured, looks like a reduced and compressed edition of the 

 large margiuata (p. 321, Plate LXIV). It maybe a descendant therefrom; and 

 yet it may be questioned whether, as the descendant of marginata, it would have 

 lost its spines so much, have become so compressed, and yet be so evolute. 



Still it must be noticed that, as regards suture-line, costata has a very 



