ACROCHORDICERAS, EUTOMOCERAS. 235 



abdomen, over which the piliie do not pass, shows that this is a 

 different genus, characterized by a different mode of development. 

 The septa are quite similar to those of Trachyceras, but it is 

 very evident that in the Trachyceratidse the septa cannot be 

 looked to for generic differences. Great differences also occur 

 in the amount of involution of the different species and in the 

 development of their external characters. 



G. ROTELLIFORME, Meek. PL 105, figs. 592, 593. Trias. 



Nevada. 

 TRACHYCERAS, Laube. 



T. WHITNEYI, Gabb. PL 105, figs. 590, 591. Trias. 



Nevada; California. 

 Family PHYSANOID^E. 



ACROCHORDICERAS. Hyatt. This genus is closely allied to 

 Lytoceras and Phylloceras, Suess, and Haploceras of Zittel, 

 combining characteristics which are found in all of these, besides 

 having peculiar characters of its own, and a different develop- 

 ment. The extent of involution is comparable with that of 

 Haploceras, but the whorl itself is about intermediate between 

 the extreme roundness of Lytoceras and the more flattened sides 

 of Phylloceras. Its peculiar characteristics consist in having 

 large lateral tubercles and abdominal pila?, which are united as 

 they near the tubercles. The smooth zone along the centre of 

 the abdomen in the young is also probably of generic value. 



A. HYATTI, Meek. PL 106, fig. 594. Trias. Nevada. 



Genus EUTOMOCERAS, Hyatt. 



This is a well-marked type, characterized \>y its lenticular 

 form, narrow umbilicus, apparently at all ages very sharp ab- 

 dominal keel, without furrows or lateral ridges, and small, regu- 

 lar arched pilse on middle sized specimens, growing wider, more 

 irregular, less distinct, and developing small lateral lobes on the 

 adult, with both nodes and pilae becoming obsolete on the larger 

 part of the body-volujbion. 



E. LAUBEI, Meek. PL 106, tigs. 595, 596. Trias. Nevada. 



The family relations of the above, as well as of the following 

 genus are not indicated. 



