Hyatt.] 10 [January 17, 
According to Oppel! this species is identical with part of Amm. 
armatus densinodus of Quenstedt, and is found in the Raricostatus- 
bed of the Lower Lias. This agrees admirably with its affinity for 
Deroceras Dudressieri, which occurs in the next bed but one below 
the Obtususbed. Quenstedt (Der Jura, p. 105) also places this 
species with raricostatus in the Lower Lias. ‘There are, however, a 
large number of specimens labelled “ Venary pres de Semur, Middle - 
Lias,” by Boucault, and these are associated with others which he 
has named “ subarmatus,” but.which are merely the broad Deroceras 
Dudressieri-like variety described above. 
Deroceras nodogigas. 
Percnoceras nodogigas Hyatt, Bull. Mus. Comp. Zoology, no. v, 
p- 85. 
Amm. nodogigas Quenstedt, der Jura, p. 125, pl. 15. 
The Museum possesses one large, but indifferently preserved spec- 
imen of this species, which does not enable me to throw much light 
upon its affinities. Quenstedt asserts that it is distinct from Oppel’s 
Amm. armatus, which appears to be plainly enough indicated by this 
specimen. ‘The pile and spines have none of the irregular swollen 
and divided aspect of those of the typical armatus, but are closely 
similar to those of Deroceras muiicum. ‘They are also more numer- 
ous than in Deroceras armatum. 
SERIES CYCLOCERATINA. 
This series * exhibits some remarkably curious and interesting fea- 
tures. Like the Dactyloide it does not increase the involution of 
the whorls, but merely the flatness of the sides, the narrowness and 
prominence of the abdomen. The approximation of the earliest 
existing species, Platypleuroceras brevispina to Microderoceras 
Birchu, is very marked, aud very curiously the adult is almost identi- 
cal with the adult of Microceras latecosia. From this species to the 
next but one, Cycloceras Valdani, is a considerable step. Not 
only has the character of the abdomen entirely changed from a 
rounded planicostan form to a keeled outline, but the young display 
this characteristic at an early age. The remaining species simply 
1 Die Jura formation. Jahrshefte fur Naturk. in Wiirt., 1856, p. 209. 
* This series was formerly described as a separate family in Bull. Mus. of Comp. 
Zoology, cn account of the differences of the adults. 
