PLACENTICERATID. 221 
PLACENTICERAS WHITFIELDI n. sp. Hyatt. 
Pl. XLV, figs. 3-16; Pl. XLVI; Pl. XLVI, figs. 1-4. 
Placenticeras placenta Meek (pars), 1876, Mon. U. S. Geol. Sury. Terr., Vol. IX, 
pl. 24, fig. 2. 
This species can be distinguished from true P. placenta of New Jersey 
and the supposed western members of the same species by the following 
characters. The highly compressed volutions are more involute and the 
venter is narrower throughout life and less completely rounded in old age, 
and this change comes in only at a much larger size than in the shells of 
P. placenta. Owing also to the absence of the median lateral line of tuber- 
cles the volution has flatter sides. All tubercles are wanting in typical 
forms at all stages, but very fine tubercles are present on either border of 
the venter and larger ones on the umbilical shoulders in some shells. 
Sometimes minute tubercles are present on the median part of the lateral 
aspect, but these occur only in the neanic stage, disappearing with the 
ventral tubercles in the ephebic stage. The sutures are more complicated 
in the young and are more overlapping than in placenta. The saddles are 
almost linear because of the excessive development of the lobes, which are 
very long and narrow. These differences hold with the materials so far 
examined. It must be remembered, however, that as yet no examination 
of a large number of specimens of both species from the same locality has 
been made, and it is likely that there are intermediate shells. Certainly no 
one ean distinguish these species unless familiar with both forms or having 
both for comparison. The same may be said of P. intercalare, between 
which and this species there are intermediate shells in P. whitfieldi vay. 
tuberculatum. 
I made special examination of the ventral lobes of whitfieldi in all 
available specimens. All had the peculiar very broad ventral lobes with 
long narrow branches on the lateral aspects except in rare cases in which 
syrtale-like or blunter arms were present. One specimen had the long 
narrow arm on the right side and a blunt syrtale-like arm on the left side. 
As arule the ventral lobe is symmetrical, but the siphonal saddle is often 
unequally developed or out of place. This saddle is often entire and flat, so 
that one is apt to regard this as the normal form, but variations are so 
frequent that only large numbers of specimens could determine the facts. 
