ACANTHOCERAS. 263 



As an aberrant form, Hopl. regularis, remains to be mentioned, 

 which diverges from all other Hoplites in the less number of 

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

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

 researches are necessary. 



Forty-five species. 

 HOPLITES ARCHIACIANUS, d'Orb. T. 112, figs. 668, 669. 



Genus ACANTHOCEKAS, Neuimijr. 



From Hoplites, near its origin, a large series diverges, which 

 perchance embraces the groups : Angulicostati, Crassecostati, 

 Nodosocostati, Mamillares, and Rotomagenses, and which, after 

 long consideration, 1 separate as an independent genus. Isolated 

 forms are found, which combine the characters of Acanthoceras 

 and Hoplites without approaching the point of divergence (origin) 

 of either. 



The diagnosis of the genus Acanthoceras ma3^ be presented in 

 the following manner : 



Successors of the group of Hoplites abscissus, with a moder- 



ely wide umbilicus and not very elevated whorls. Margin of 

 rture 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- 



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

 etiines with uninterrupted ribs, sometimes with a furrow, 



>metimes 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 tianks there is at most one auxiliary, or 

 a row of two to three extremely small deep-lying auxiliaries \ 

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

 broader than the first, 110 branching, but only a dentation of the 

 lobes. Siphonal and first lateral usually not very different 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. 



:;:', 



