1870.] 27 [Hyatt. 



In Deroceras Dudressieri this process is repeated at about the same 

 period, but the dividing cell does not reach a similar prominence, nor 

 do the septa in general terms become quite as complicated as those 

 of the adult Microderoceras Birclrii until a much later period. Thus, 

 while the lobes and cells of the former have become almost as com- 

 plicated as in the adult, on the last quarter of the fourth whorl, those 

 of Deroceras Dudressieri are a full volution later in reaching the 

 same stage, and are never so deeply cut or foliaceous even in the 

 adult as in the adult of Microderoceras Bircliii. 



The first stage in the development of the latter corresponds to one 

 which occurs in a precisely similar manner in Deroceras planicosta, 

 but not until that species nearly reaches the completion of its fourth 

 whorl. In Deroceras confusum there is no constancy in the de- 

 velopment of the minor cells. Two opposing cells may be brought 

 out unequally, as in the young Birclrii, or symmetrically, or only one, 

 invariably that from the side of the superior lateral cell. In other 

 words, the adults have all the modes of division found in the different 

 stages of growth of Birclrii, according to the stage at which arrest 

 of development has occurred. In neither Deroceras planicosta or 

 Deroceras confusum do the septa reach a stage of complication com- 

 parable with any but the youthful stages of Deroceras Dudressieri and 

 Deroceras Birclrii. D. ziplrius was not examined, but the septa prob- 

 ably accord with the growth of the external ornaments and pilaa which 

 place it near D. Dudressieri. The condition of D. Dudressieri and 

 D. ziplrius in the adult stage corresponds in their single external line 

 of spines and rounded abdomen to the early stage of M. Birchii, 

 before the internal line of spines is brought out; that of the adults 

 of Deroceras planicosta and D. confusum to the young of these two 

 species when the spines are developed, and the abdomens still have 

 the planicostan folds. This characteristic, it will be remembered, 

 occurs also in some specimens of Microderoceras Birclrii, but is only 

 faintly expressed ; in Deroceras Dudressieri and Deroceras ziplrius it 

 is constantly expressed in the young, to a later period in the former 

 than in the latter, and is of constant adult value in Deroceras plani- 

 costa and Deroceras confusum. The inference seems to be unavoid- 

 able that the species of this series, which occur later in time and are 

 all smaller than Microderoceras Birclrii, are really dwarfed and de- 

 graded descendants of this comprehensive species. 



Considering the septa in the next series, we have first Microce- 

 rqs biferum. The superior lateral lobes in this species constantly 



