Hyatt.] 8 [Jaauary 17, 
the lobes and cells show that there can be no doubt of the affinity of 
this species for Microderoceras Birchii; but the single row of promi- 
nent spines indicates that, like Deroceras Dudressieri, it is a generic 
derivative from Microderoceras Birchi. And an examination of the 
young septa shows that at one stage they have the large cell in the 
centre of the superior lateral lobes so common in Deroceras Dudres- 
siert. Thus the species is properly and more directly a descendant 
of Deroceras Dudressieri with reversions to Microderoceras Birchiu. 
The absence of a planicostan abdomen at all stages places it in 
advance of Deroceras Dudressieri. The geological position of the 
latter in the Lower Lias Obtususbett, and of this species on the bor- 
ders of the Middle Lias, confirm this view. 
Deroceras alternum. 
Peronoceras alternum Hyatt, Bull., Op. cit., p. 85. 
The young of this species is smooth, like the young of Deroceras 
armatum, the tubercles and pile are developed slowly, as in that spe- 
cies, and the characteristics of the adults are similar, with fold-like, 
spined pile, and numerous crenulations between. The abdominal 
cell is exceedingly large. The abdominal lobe is very broad, and 
much deeper than the superior lateral lobes. The superior lateral 
cells are extremely narrow, and much cut into by the minor lobes. 
The superior lateral lobes have three wide spreading branches and 
the intermediate cells therefore are very large, and the inferior lateral 
cells very much attenuated at the top. These characteristics approxi- 
mate also to those of Deroceras muticum, though not so closely as to 
those of Deroceras Davei, while the shell is more like Deroceras 
armatum. It possesses just the same intermediate character as Dero- 
ceras Davei, but is a very much smaller species, with the young 
more like Deroceras armatum. 
Deroceras Davei. 
Amm. Davei Sow., Min. Conch., vol. 1v, p. 71, pl. 350. 
Deroceras Davei Hyatt, Bull. Mus. Comp. Zoology, no. v, p. 84. 
This species carries to excess the sparse distribution of the spines, 
the mere folds instead of pile, and the multitudinous crenulations ob- 
served in Deroceras armatum. The spines unite these when they 
occur in knots, but this is subject to the greatest variations, and the 
number of the spines varies considerably. ‘They are not present in 
the young until a late period, and then occur pretty regularly, though 
much wider apart in some specimens than in others, and finally dis- 
appear suddenly as old age approaches. The advance of age is 
