ACANTHOCERAS. 263 



As an aben-aiit form, Hopl. regularia, remains to be mentioned, 

 which diverges from all other Hopliten in tlie less number of 

 lobes which it possesses, whilst in other respects it is closely 

 allied to the true Dentata; in regard to its true relations, further 

 researches are necessary. 



Forty-live species. 

 HOPLITES ARCHIACIANUS, d'Orb. T. 112, figs. (itiS, (iH9. 



Genus ACANTHOCEKAS, Neumayr. 



From HoplUes, near its origin, a large sei'ies diverges, which 

 perchance embraces the groups : Angulicostati, Crassecostati, 

 Nodosocostati, Mamillares, and Rotomagenses, and which, after 

 long consideration, I separate as an independent genus. Isolated 

 forms are found, which combine the characters of Acanthocei^as 

 and Hoplitea without approaching the point of divergence (origin) 

 of either. 



The diagnosis of the genus Acanthoceras may be presented in 

 the following manner : 



Successors of the group of Hoplites abscissus^ with a modei'- 

 ately wide umbilicus and not very elevated whorls. Margin of 

 aperture and length of body-chamber unknown. The sculpture 

 consists of quite straight ribs, which become constantly stronger 

 from the suture outwards to the external side, which are fre- 

 quently ornamented with a greater or less number of tubercles 

 or nodes, and are most curved in young individuals. The de- 

 velopment of the external side is very variable, the middle line 

 sometimes with uninterrupted ribs, sometimes with a furrow, 

 sometimes with a line of tubercles, the elements of which attempt 

 to unite into a keel. Lobular line much reduced ; besides the 

 two laterals on the flanks there is at most one auxiliary, or 

 a row of two to three extremely small deep-l3'ing auxiliaries ; 

 bodies of the lobes and saddles plump and broad, the last 

 broader than the first, no branching, but only a dentation of the 

 lobes. Siphoual and first lateral usuall}' not verj' diflerent in 

 size, the first often larger than the last ; second lateral much 

 smaller than the first, both one-pointed. 



Thirty-six species. 

 Acanthoceras Rotomagense, Brong. T. 103, fig. 563. 



