HAMITES. 247 



the establishment of a superabundance of genera ; after careful 

 trial I believe that the following may be included in Hamites : 



Anisoceras, Ancyloceras,* Baculina,* Hamulina, Helicoceras* 

 /'.'t/choceras, Toxocerax. 



The principal reason why I am necessitated to suppress these 

 genera, is this : that in their characterization, only the charac- 

 ters prevalent in the species of the series belonging here are 

 used, a proceeding by which, of course, a completely unnatural 

 subdivision is effected. In the departure from the closed spiral, 

 a new direction of variation is assumed, and one is therefore en- 

 tirely justified in making a division here from the old stems; 

 for farther subdivision, on the contrary, we must adopt few or 

 no characters derived from the tendenc}^ of variation. The 

 sculpture is here hardly available, and indeed hardly at all in 

 the beginning of the series of evolute forms, but perhaps in the 

 farther stages, where an abnormal development and strengthening 

 of ornamentation tends to take place. In this respect, the lobes 

 will best serve our purpose, since we find amongst evolute forms 

 a great number which present exactly the symmetrical structure 

 of the lobes of Lytoceras, whilst the others have equally dis- 

 tinct unsymmetrically divided lobes and saddles. 



Amongst the forms which present symmetrically divided lobn- 

 lar structure, are some, the geologically oldest, which also ex- 

 hibit such striking agreement with Lytoceras, that there can be 

 no doubt that these have descended from representatives of that 

 ;nus. Aside from the relations of the whorls, all other charac- 

 M'istics of Scapliites Yvanii, further of Crioceras Astierianum 

 id depressum, these agree most completely with cretaceous 

 /toceratidae, the first with L. rectecostatum, the last with the 

 ;*oup of Lyt. Timotheanum. By simple continuous develop- 

 icnt of the spiral in the direction of variation, and of course 

 >rogressing quite in the normal way from without inwards, we 

 )btain from Scaphites Yvanii the genus Hamites, from which 

 Ptychoceras differs only in the most subordinate characters. The 

 imperfectly known genus Anisoceras may also be most appro- 



* The cretaceous forms of Ancyloceras and Baculina must be embodied 

 into other genera, though one could easily transfer their names to forms 

 from the middle Jurassic, for which otherwise new names would have to 

 be coined (Ancyloceras callomense and annulatum, Baculina cenaria). 



