192 PSEUDOCERATITES OF THE CRETACEOUS. 
resembles that of some of this genus in having a channeled venter, but 
it is much too involute and compressed for a primitive type. The keel 
of balduri is developed in the center of a broad, concave venter of neanic 
age, as an obtuse raised area, but finally the entire venter becomes 
elevated with a subacute carina, like that of Buchiceras. <A raised nascent 
keel may be said to appear at a late age in some species of Placenticeras, 
but the venters in them are very narrow, and no such effect is produced 
as in balduri. The very narrow channel of the ananeanic stage in this 
species and the late development of the tubercles produce resemblances 
to Diplacmoceras, but in this genus the outer lines of tubercles are not on 
the ridges of the venter as they are in balduri. If this species is the 
ancestor of Diplacmoceras, we have still to account for this and the fact 
that the nepionie stages in Placenticeras exactly match those of Protengono- 
ceras and Knemiceras and are never keeled as in Buchiceras and Roemeroce- 
ras. So far as Keyserling’s drawings go, the nepionic form of balduri 
before the channel appears is like that of Buchiceras and Tissotia serrata. 
The separation of the genus Placenticeras from the Hoplitide “ demands 
a few words of explanation. Its association with Hoplites by Douvillé and 
Grossouvre depends upon the connection supposed to be shown by the 
large first lateral saddle of Hoplites splendens. 'Vhis saddle, as figured by 
these two distinguished authorities, has the three large marginal lobes and 
three saddles which are supposed by them to be homologous with the three 
principal laterals of Placenticeras. 
The author’s position is quite distinct from this. If the three marginals 
of H. splendens, having undoubtedly, as stated by them, | great similarity to 
the principal laterals (first to third) of Placenticeras, are intermediate grada- 
tions and not a case of parallelism, some similar stage ought to be present 
in the development of species having three principal laterals. This, so far 
as I know, is not the history of the development in any form of this kind. 
@1t is disappointing and much to be regretted that no direct mention is made of Prof. James 
Perrin Smith’s paper on The Development and Phylogeny of Placenticeras (Proc. Cal. Acad. Sci., 3d 
series, Geol., Vol. I, pp. 181-240), although incidental reference to it is made in connection with 
the development of the sutures. If Professor Hyatt had completed the revision of his manuscript, he 
would doubtless have reviewed this paper and made some dispositon of the two species P. pacificum 
and P. californicum, treated in it. Professor Smith concludes that Placenticeras was derived directly 
from JHoplites, but the two species whose ontogeny he studied differ greatly from the typical forms of 
Placenticeras, and the results should be tested by a comparative study of some such forms as P. placenta 
or whitfieldi.—T. W.S. 
