Hyatt.] 26 [October 5, 



The superior lateral lobes of Microceras biferum always, even in the 

 young, seem to possess a median, minor cell which is absent in 0. 

 raricostatum. The latter species is much the largest, and the adult 

 septa differ widely. No genetic connection is traceable in their de- 

 velopment except in very general terms. On the other hand, the 

 affinities of M. biferum in all respects point them out as degraded 

 and dwarfed descendants of Microderoceras Birchii, which precedes 

 them, also, in time. 



There are other forms, however, which render these questions still 

 more puzzling. A series of single spined or armatus-like species be- 

 gins with Deroceras planicostatum, Dudressieri and Deroceras zipliius 

 in the "Obtususbett," and is continued by Deroceras confusum in the 

 " Raricostatusbett." The development of Deroceras armatum does not 

 join it directly with any of these species, and since it occurs only in 

 the lower bed of the middle Lias it need not be considered in this 

 connection. 



Deroceras Dudressieri has the planicostan abdomen in the young, 

 but in the adult possesses the abdomen of Microderoceras Birchii, and 

 in fact differs from that species at this stage principally by the ab- 

 sence of the inner line of spines ; the septa are very similar in both. 

 Deroceras zipliius differs more widely from Microderoceras Birchii 

 than Deroceras Dudressieri, but in features which it is not important 

 to discuss here. Then we have Deroceras planicosta, which never 

 parts with the typical planicostan abdomen, though in the adult it 

 acquires a single row of spines, as in Deroceras Dudressieri \ and lastly, 

 Deroceras confusum (Amm. Lohbergensis Emerson), which differs 

 somewhat from D. planicosta in the septa, but more in the slighter 

 form of the whorl. 



If, now, we examine closely the development of the septa in Micro- 

 deroceras Birchii, we find that it equally resembles the development 

 of the septa in all of the members of the two series just described, 

 which exhibit the planicostan abdomen largely in their growth. The 

 septa of Microderoceras Birchii on the first quarter of the third 

 whorl acquires three minor cells, and the superior lateral lobes 

 become divided, first by the rise of minor cells from the sides of the 

 superior lateral cells. During the same stage a very minute crenu- 

 lation becomes developed from the side of the inferior lateral cell; 

 this, however, does not increase as fast as its opposing cell, which 

 eventually reaches a very large size, equally dividing the superior 

 lateral lobes. 





