82 INFERIOR OOLITE AMMONITES. 



carina are visible, even over the air-chambers (but see foot-note). The inner 

 margin is upright, well-developed, and straight. The inclusion seems to be 

 always considerable, and scarcely any portions of the inner whorls are exposed. 

 The umbilicus is small, rather deep, formed of a series of very small steps ; it has 



chamber in some species the carina, after the hollow tube has ended, continues to be of the same size, 

 and the core, which was formerly non-carinate, now exhibits a large carina; that is to say, the animal 

 in that part of the body-chamber occupied the carina without the intervention of a partition. In other 

 cases, after the termination of the hollow tube the carina decreases in size. 



It is important that these facts concerning the change in the ventral area of the body-chamber 

 from a hollow-keeled to a solid-keeled state should be correctly understood, because otherwise an 

 examination of the carina of the body-chamber might lead to the erroneous supposition that the whole 

 Ammonite was necessarily solid-keeled. It is therefore possible for the same specimen to exhibit 

 both solid-keeled and hollow-keeled states ; but for that to be apparent a considerable portion of the 

 body-chamber must be present. 



From the above remarks it will be seen that we have a most important and peculiar structure in 

 the carina of certain species of Ammonites. What its functions may have been I cannot say. Whether 

 the partition -band is part of the inner shell, or a separate growth, is uncertain ; but I incline to the 

 latter idea, because the partition-band separates cleanly away from the test, and is often present when 

 the test is wanting. In many genera the hollow carina is a most conspicuous feature, notably in 

 Haugia, Harpoceras, Sfc, and the ventral area of the core is rounded ; in other genera, such as Pseudo- 

 lioceras, the feature is not so conspicuous, and the ventral area of the core shows a very slight indication 

 of a carina ; while in Witchellia 1 the hollow carina varies somewhat in size, and the ventral area of the 

 core shows a slight carina bordered by two furrows which are not seen on the test. 



The value of the hollow carina for the purposes of classification is uncertain. When I first 

 noticed the feature I hoped that it would enable me to make some very clear definitions. This hope 

 must, I am afraid, be abandoned. It would seem that this structure is to a certain extent of generic 

 value since it enables us, for instance, to separate Pseudolioceras from Lioceras ; but it does not assist 

 us to distinguish Haugia from Harpoceras. If Ammonites are to be classed according to their descent, 

 it seems to be impossible to keep the hollow-keeled Ammonites in one family and the solid-keeled in 

 another ; because, according to my reading of the facts, the hollow-keeled are sometimes descended 

 from solid-keeled, and vice versa. Should we, however, reject the theory of evolution entirely, we 

 could not arrange the Ammonites as the solid-keeled family and the hollow-keeled family without doing 

 violence to what is expressed by similarity of other structures, such as ribs, suture-line, &c. ; and we 

 should find that in the one family were included Ammonites, the most diverse in these respects, but 

 agreeing only in possessing a hollow carina; while those Ammonites which were excluded from this 

 family differed in no way from certain included members except in this matter of keel-structure. 



Taking all these points into consideration, I can only conclude that, functionally, the hollow 

 carina was not of any great service to the Ammonite ; that it was a character which, perhaps by 

 disuse, became undeveloped in certain species without any detriment to them, or again was developed if 

 circumstances so required. I cannot at present see my way clear to make any use of it in classification 

 except so far as genera are concerned. I am even doubtful if I should venture to create a genus 

 merely on the presence or absence of a hollow carina in the species ; but hitherto I have not had to 



1 Witchellia, nov. gen., dedicated to the memory of my kind friend Mr. E. Witchell, F.Gr.S. 

 The type is Witchellia Iseviuscula, Sow. sp. ; and to this genus belong also Am. jugifer, Waag., Am. 

 Sutneri, Branco, Am. deltafalcatus, Quenstedt. 



