Hyatt.] 232 [December 2, 



ened, but the interior evidently hollow, while in the French speci- 

 mens at Semur the shell was of the usual thickness. The former may 

 possibly indicate a transition to O. oxynotum. O. Guibalii, however, 

 by the resemblances of the young to the adult of 0. Guibalianum, or in 

 other words by the younger period at which the adult characteristics 

 of the species are inherited, is apparently a direct derivative from 0. 

 Guibalianum. The same reasons would also apply to O.Lotharingum, 

 in which the young lose the striaries-like stage almost entirely and re- 

 peat only the adult form and characteristics of 0. Guibalianum. The 

 shorter and shorter duration of the adult stages and the continually 

 earlier period at which the senile stage of decline makes its appear- 

 ance in each successive species probably indicate a similar relation- 

 ship. The evidence of the descent of these three species from each 

 other would be complete if they did not all make their appearance on 

 the same level, in the Birchii or Obtusus bed at Semur. As the case 

 now stands, therefore, no relation of succession in time can be af- 

 firmed. The relations of the different species would seem rather 

 to be represented by divergent lines connecting 0. oxynotum with 0. 

 Guibalianum through a series of intermediate forms, and then 0. 

 Guibalianum with Guibalii and Lotliaringum, hut all on the same geo- 

 logical level. 



Oxynoticeras oxynotum Hyatt. 



Amm. oxynotus Quenst., Petrfk. 98, pi. 5, f. 11. 



This species presents us, in the course of its development, with some 

 curious and interesting metamorphoses. The young are round and 

 smooth and may continue to retain this smoothness and the rotundity 

 of the abdomen until the specimen is fully an inch in diameter. The 

 sides, however, become flatter and slight folds and striations become 

 visible. The resemblance to Agassiceras striaries is so decided in 

 these specimens that if found independently, no one would hesitate 

 to place them in the same genus as a closely allied species. The 

 septa even agree very closely, as may be seen in Prof. Fraas' collec- 

 tion, which presents a very fine series of these exceptional varieties. 

 The keel does not appear until much later in these striaries-like forms 

 than in the normal forms, but in all varieties the Arietian characteris- 

 tics of the septa are apparent. The keel on its first appearance 

 seems to be solid, though I could not determine this with absolute 

 precision. If this could, have been determined, the evidence that this 

 species is a descendant of Agassiceras striaries would have been com- 

 plete. Besides this variety, there is another evidently normal and 



