140 INFERIOR OOLITE AMMONITES. 



ever, no actual need for them on account of having obtained, for some reason or 

 other, less side-breadth to support ; and, in order to accommodate them, it would 

 appear that it has had to decrease the size of its other lobes. 



All the species of Hyperlioceras show a most decided tendency in adult age to 

 suddenly become, when the body-chamber is present, widely umbilicate. 1 This 

 character is also found in Lioceras in a less degree, but is almost absent, I believe, 

 in Pseudolioceras. 



Before proceeding to notice the species of the next group, I may remark that 

 the very small fry of the foregoing genera (in all cases, I believe) not only differ 

 from the adults by being evolute, with rounded whorls, but commence their 

 ribbing in the first instance as straight folds on the sides of the whorls. So far 

 as their shape and ribbing go, the development from youth into adult of a 

 highly-developed species follows about the same rules as those which, as I have 

 been trying to show, hold good in connection with the evolution of these species 

 from Arietiies. In fact, they fully illustrate the truth of my Rule III, p. 134. 



The species which belong to Branch C show, as far as the Inferior Oolite is 

 concerned, a much wider gap between themselves and the Inferior- Oolite repre- 

 sentatives of Branches A and B. In fact, the representatives of Branch C are 

 found at a much earlier period, namely, in the Jamesoni-zone, where they have 

 already obtained a thoroughly distinctive appearance. A development somewhat 

 similar to what they have reached at this early date is only attained by the 

 representatives of Branch B during the Middle- Lias period, and by the representa- 

 tives of Branch A during the Upper Lias. At the same time Branch C has 

 developed but very little since the Jamesoni-zone, so that the species which we 

 meet with at the end of the Upper Lias exhibit characters very similar to those of the 

 early species. As a consequence of what is practically a cessation of development, 

 few generic divisions are required in this branch when compared with the others. 



The earliest species is Grammoceras antiquum (Wright) in the Jamesoni-zone ; 

 and this exhibits already considerable development, namely, compressed, broad 

 whorls, and the absence of the ventral furrows. Excepting some species in the 

 Margaritatus-zone, we do not, in reality, meet with any quantity of forms until we 

 reach the borderland between Lias and Oolite. One of the species which is found 

 at this point, namely, Grammoceras striatulum, appears to be far less developed than 

 the older Grammoceras antiquum ; but it will probably be better to defer a detailed 

 criticism until I have described the various species. 



It is interesting to notice that certain species of this branch exhibit marks of 

 their descent. A variety of Gramm. Normannianum has ventral furrows ; so has 

 Gramm. ovatum; while certain other species exhibit rudiments of the same. 



1 This character is also shown by GEcotraustes and some species of Oppelia ; while Hammatoceras 

 fallax, and Sphseroceras, add to it a contraction of the thickuess of the body-chamber. 



