CARBONIFEROUS CEPHALOPODS. 349 



that the former retains throughout life — that is to say, on all parts of its larg- 

 est whorl, which is much larger than that of the European species — the pecu- 

 liar but flattened abdomen which is found only in the young of Phacoceras 

 Rouilleri. This character is of genetic importance, and, together with the 

 longitudinal ridges and form of the young in this species, and in P. oxysto- 

 mum, show that these acute involute shells were derived by descent from 

 more discoidal shells like those of the genus Discitoceras. This also serves 

 the purpose of explaining the occurrence in the Carboniferous of their appa- 

 rently anachronic forms and structural characteristics. The aspect of the 

 adults and the sutures in this genus are like Triassic species such as Grypoceras 

 (Nautilus) galeatus, Mojsisovics, and at first they appear to have occurred be- 

 fore their proper geologic period. When, however, their young are studied, 

 it is plain that their shells at early stages have the ordinary characteristics of 

 normal members of the Carboniferous faunas, and that the peculiarities of 

 later stages were evolved from purely Carboniferous forms. Their mimicry 

 of Triassic shells in later stages must therefore be regarded simply as good 

 examples of parallel progressive complications arising independently in differ- 

 ent genetic series during different periods of time. In Rouilleri the flattened 

 aspect of the crest of the abdomen is retained much longer in the course of 

 the growth than in Phacoceras oxystomum. The American species, with its 

 truncated abdomen existing in the adult, is therefore the most immature 

 form of the group yet discovered, and although it is as yet impossible to 

 come to any conclusion, this fact at present points to the fauna of this coun- 

 try as the place of origin or aldainic fauna of this series. Rouilleri is prob- 

 ably genetically connected with P. JUumbli, or -some equivalent species, and 

 P. oxystomum is similarly connected with P. Rouilleri. In both of these, 

 however, it is superseded in the subsequent stages of shell growth by an 

 acute abdomen. 



