PLACENTICERATID &. 245 
smallest fossil, about 10 mm. in diameter, was exactly represented in Key- 
serling’s figure which shows the acute venter. Neumayr“ does, however, 
state clearly that the venter becomes furrowed at 15 mm. in diameter, and 
that this furrow is succeeded by a stage with a rounded veuter which pre- 
ceded the incoming of a stage with a slightly convex venter and distinct 
ventro-lateral angles, and that this precedes a stage with distinct keel devel- 
oped upon the elevated venter, and tubercles developed upon the costee 
internally and at their terminations on the ventro-lateral angles. Such an 
extraordinary succession of transformations is irreconcilable with those of 
the ontogeny of any group known to the author. The resemblances of the 
sutures are, therefore, not sufficient to place the genus in the same group 
with Platylenticeras, from which it differs in every other respect. 
Neumayr’s view of the affinities of this genus for the Amaltheide is 
not sustained by anything except the general similarity of the external 
aspect of adults, which is probably due to parallelism, and the same is true 
of its supposed affinities for Cardioceras. The characteristics of form and 
ornamentation supposed to be so similar have obviously arisen from entirely 
distinct modes of development, and these genera are not even as closely 
related to each other as to Styracoceras. 
Age: Neocomian. 
@Ueber Amalth. balduri Keyserl. u. d. Gattung Cardioceras. Neues Jahrb. fir Min., Geol., and 
Pal., 1886, 1, p. 95. 
