1872.] 9 [Hyatt. 
indicated not only in this way, but by the irregularity of the crenu- 
lations. ‘These, at intervals, are almost obliterated, and then grow 
gradually more prominent, again dropping suddenly to mere lines. 
Undoubtedly this indicates a failure of the vital powers, and a much 
slower growth of the shell. Longer and longer arrests of growth 
explain the increasing prominence of the crenulations, and a renewal 
of energzy by means of these long rests may possibly explain the 
sudden smoothness; the animal affected by senility not being able to 
build and retain the crenulations, when the old rate of growth is 
suddenly resumed. The septa resemble closely those of Deroceras 
muticum. The superior lateral cells are, however, wider, the superior 
lateral lobes deeply divided, the inferior lateral cells very narrow. 
The abdominal lobe is deeper than the superior laterals. 
Thus while more like Deroceras muticum than Deroceras armatum 
in its septa and in the external characteristics of the shell, still the 
folds, instead of pile at each spinous node, the spines and the crenu- 
lations between them, are characteristics uniting it with Deroceras 
armatum. ‘The affinities therefore are doubtful, though in my opinion 
it is nearer to the latter than to the former, and is probably a spe- 
cies in the same series as Deroceras armatum. 
Deroceras muticum. 
Peronoceras muticum Hyatt, Bull. Mus. Comp. Zoology, no. v, p. 85. 
Amm. mulicus D’Orb., Terr. Jurass., Ceph., pl. 80. 
This species has two well marked varieties, one closely resembling 
Deroceras Dudressiert in the breadth of the abdomen and the di- 
vergency of the sides; the other peculiar, with flat, or slightly con- 
vergent sides. The pile and lower portion or base of the spines 
are not, as in Deroceras Dudressieri, filled with shell, and they are 
very distinct on the casts; nor do the young or adults even, so far as 
I have seen, exhibit the planicostan abdomen. The young, in other 
respects, are very similar to the young of that species, but the pile 
and the spines appear at an earlier age. 
D’Orbieny’s figure of the septa is excellent, and by comparison 
with those of Deroceras Dudressieri many constant differences can be 
observed, thouch the general characteristics are similar; the narrow, 
prominent, abdominal cell, the deep divisions, or minor three lobes at 
the bases of the superior lateral cells, the huge lateral cells on either 
side of the tops of the superior lateral lobes, and the extreme nar- 
rowness of the aperture at the top of the inferior lateral cells. 
