1872.] 5 (Hyatt. 
This species in the communication referred to was shown to be the 
probable ancestor of a series of forms,which include Deroceras con- 
fusum and Deroceras densinodum. All of my observations, as well 
as those of Quenstedt, indicate that the true position of Deroceras 
densinodum, is between Deroceras Dudressieri and Deroceras arma- 
tum, and its geological relation is similar. It is found in the Ra- 
ricostatusbed, just between the Obtususbed containing Deroceras 
Dudressiert and Deroceras planicosta, and the Armatusbed of the 
Middle Lias. My material, however, does not permit me to com- 
pare the young of Deroceras armatum and Deroceras densinodum in 
order to ascertain whether the former is really more closely allied to 
the latter than to Deroceras Dudressieri, but Quenstedt supplies this 
deficiency in the proof. According to his figure, this species is really 
an armatus, in which the young is pilated or ribbed at a very early 
period as in Dudressieri, instead of being smooth as in armatus proper. 
The general tendency of the series, as shown by the adults and 
by the growth of individuals, is to produce the spines at wider inter- 
vals, depress or obliterate the pile, and by decreasing the breadth 
of the abdomen render the whorl more cylindrical. Deroceras 
Dave carries this tendency still farther. The young being smoother, 
the spines more sparsely distributed, the fold-like pile almost 
absent in some specimens, and wholly superseded by the projecting 
lines of the transient mouths. These having previously been 
merely subordinate pile, have become primary and replaced the 
folds. It is not at all improbable that Deroceras alternum, from 
Plateau de Lazac, which appears to be very closely allied to Deroce- 
ras Davei, may prove to be a local variety of the latter. However 
this may be, the young are more like those of armatus than are 
those of Davai. Deroceras minatum is labelled by Dr. Krantz 
as a new species from the Upper Lias of the Plateau de Larzac. 
Tt differs in the form of the adult whorl and in the septa, though 
not in the young, from Deroceras alternum of the Middle Lias. 
The sides and abdomen are flattened instead of gibbous, as in the 
latter, and the shell is much flatter and thinner. The lobes are more 
deeply divided, and larger every way in Deroceras alternum, and the 
cells narrower at the neck, and spreading more at the base than in 
Deroceras minatum. In neither of these species are the specimens 
probably fully matured, but yet in one specimen of Deroceras alter- 
num, only a half inch in diameter, the changes are fully as extensive, 
and the septa as completely developed as in the adult of Davei. 
