1872.] if (Hyatt. 
indicate that there is a tendency existing in the organization to flat- 
ten the sides, and render them convergent instead of divergent. 
Whether this is a governing one or not in the series cannot be 
determined, and perhaps may never be, for it would not be at all 
surprising if these two species represented all that ever existed. 
Tt is well to remember, however, that this is the same tendency 
which finds a fuller expression in the series made by Dactylioceras 
commune, Holandrei, annulatum, and Braunianum. It is here, how- 
ever, only slightly shown, and the sides do not, in the most extreme 
variety, approximate, or become so convergent as to make the abdo- 
men acute, as in Dactylioceras Braunianum. 
The difference between the varieties of Deroceras muticum are 
however, comparable, so far as the divergency or convergency of the 
sides are concerned, to the difference between the divergent sides o 
Celoceras Desplacei, and the convergent sides of Dactylioceras 
Holandrei or annulatum; species undoubtedly distinct and easily de- 
finable, on account of numerous other characteristics. 
Deroceras armatum. 
Amm. armatus Sow., Min. Conch., vol. 1, p. 215, pl. 85. 
Deroceras armatum Hyatt, Bull. Mus. Comp. Zoology, no. 5, p. 95. 
This species has, from the earliest period, a very marked character. 
It is smooth much longer than the young of Deroceras Dudressieri, or 
than any of the Liparoceratide, except MJicroderoceras Birchit. 
This species it resembles closely in form also, and in the acquisition 
of the spines. Thus the young Burchw have no pile, or only 
slight swellings or ridges, and in this species this condition is 
maintained throughout life. The whorl is crossed by numerous 
ridges between the spines, and three or more of these are often gath- 
ered into knots by the spines themselves, but no true pile, such as 
appear in the adult of Microderoceras Birchii appear. ‘The sparseness 
of the spines and of pseudo-pile or ridges is also remarkable, espe- 
cially as it is found in all of this species. 
The septa closely resemble those of Microderoceras Birchii, — the 
large abdominal or siphonal cell in the centre of the abdominal lobe, 
with the adjoining superior lateral cells apparently stuck on to 
the side of the latter,—the height and extremely widespreading 
minor lobes, as well as the size of the superior lateral lobes, and the 
depth, narrowness and mode of division of the superior lateral cells 
by the very prominent minor lobes,—these decisive similarities, 
and the singularly close likeness of the very ornate outlines of all 
