72 GENESIS OF THE AKIETIDJE. 



Cal. Johnstoni was at first smooth, then ribbed, and the ribs had a peculiar 

 fold-like character, and they appeared in this succession in the young of all the 

 remaining series. The keel was added to these in the adult of Cal. tortile after 

 the pilre were developed. The adult of Cal. laqucum had the keel, and added 

 also faint channels and in one variety the tubercles. 1 If we are right in referring 

 Cul. Deffneri"" to this series because of its sutures, then the terminal species of 

 Caloceras had a very highly accelerated development, producing the quadrago- 

 nal form, keel, channels, complete pila3, and tubercles at nearly the same stage 

 of growth as in Ver. ophioides. 



In the development of the young of Ver. Comjbcari, the succession of charac- 

 ters was similar, — first a smooth whorl, then fold-like psiloceran pila?, then keel, 

 then channels, then true pila3, which often became tuberculated. In Ver. ophioides, 

 a tuberculated species with highly accelerated development, it is difficult to 

 determine whether the keel was developed before or after the channels, since 

 they appeared almost simultaneously. 



In the varieties of Cal. laqueum, we found the forerunners of all the rounded, 

 quadragonal, keeled, channelled, and tuberculated forms of Vermiceras. The 

 variety which led into Ver. spiratisdmum had no tubercles, and could be called 

 quadragonal, though it had only a slight keel and no channels, or at most 

 very faint bands of depression on either side of the keel. In Ver. spiratisdmum, 

 these characteristics are fixed within narrower limits of variation, the keel, chan- 

 nels, and quadragonal form were invariably present in adults, but better defined 

 in some than in others, and no variety with tubercles has yet been discovered. 

 In Ver. Conybeari also the characteristics above mentioned were more invariable, 

 but here we found numerous adult individuals with tubercles on the pilae, and 

 varieties which produced all these advanced characteristics, including the tu- 

 bercles in some cases, at a very much earlier age than in other species. Lastly, 

 Ver. ophioides is always tuberculated, channelled, and keeled in adults, though, as 

 shown by its young, evidently derived from those individuals of the tuberculated 

 variety of Comjbeari which did not produce these same characteristics at so 

 early an age. 



In the arnioceran series, the keel appeared in the young before the channels, 

 and also previous to the development of pilas. This is the case in Am. miserabile, 

 and in the derivative Am. semicostatum. This is in strict accord with an indepen- 

 dent descent from the smooth Psiloceratites,- but not with a supposed derivation 

 from any intermediate caloceran or plicated form of Psiloceras. The pilas ap- 

 peared in the growth of an individual of Caloceras before the keel, — a condition 

 due to the fact that the young remained, as previously explained, very similar to 

 a plicated keelless Psiloceras during the earlier stages of development. The 

 channels were first apparent in the adults of certain varieties of Am. semicostatum, 

 and in some other species of Arnioceras they became of specific value. 



1 This progression was much fuller in the species of the Mediterranean province (see Summ. PI xi. 

 fig. 17-19), showing the correlations of the development of the individual and evolution of forms in the 

 series better than in Central Europe. 



2 Summ. PI. xi. fig. 21. 



