Hyatt.] 28 [October 5, 



divided equally, as in the adult of Microderoceras BircMi] the supe- 

 rior lateral cells are divided into two unequal portions by a large 

 minor cell, and are very similar in outline to the young of Deroceras 

 Dudressieri on the fifth whorl, and to the young of Microderoceras 

 BircMi at an earlier period, while the cells are broader and less 

 deeply cut than they were observed to be upon the latter part of the 

 fourth whorl. The young of M. laticosta are precisely similar to 

 the young and adult of Microceras biferum, but the septa bring out 

 equally the opposing median cells, and the superior lateral lobes thus 

 become unequally divided. In the adults they reach a state of com- 

 plication comparable to those of Microderoceras BircMi and Deroceras 

 Dudressieri. With Microceras laticosta are associated the strongly 

 reversionary species, which only need a keel to be classified with the 

 Arietidas. This is especially the case with Microceras arcigerens, 

 whose septa, in one specimen, are remarkably similar in proportions 

 and outline to those of Aster ocer as obtusum, and what is still more 

 remarkable in this same specimen, a slightly raised siphonal line 

 is plainly apparent between the prominent planicostan folds. 1 



In Androgynoceras Tiybridum an equally complicated state of the 

 septa is reached at an early stage, and still earlier in the succeeding 

 species of Liparoceras. 



Microceras biferum is of small size, about an inch in diameter, and 

 at the latest stage assumes a double row of spines, or is smoother ; the 

 pilse in all cases closely simulating those of the adult Microderoceras 

 BircMi at this period. Thus it may be said to play the same part 

 that Deroceras Dudressieri does in the armatoid or single spined 

 series in its external characteristics and form, while in its septa it 

 corresponds to Deroceras planicosta. In the same way Deroceras 

 laticosta may or may not have the double row of spines, but never 

 has a single row, 2 and never in the adult returns to the rounded 

 abdomen and peculiar pilaa and ornaments of Microderoceras BircMi. 

 Androgynoceras, however, does return to this condition in the 

 adult, but at the same time another tendency is developed both in 



1 A close comparison with Zieten Amm. Turneri, which I regard as a variety of 

 Asteroceras obtusum, shows, however, that a discrepancy exists in the proportions 

 of the abdominal lobe and in the remaining general characteristics of form, which 

 do not permit any attempt to trace a direct genetic connection. 



2 Microceras biferum occasionally has a broad projection on the pilas which 

 might be mistaken for a single spine, whereas it is really formed by the coalescing 

 or arrested development of two rows, 



