PLACENTICERATID®. 223 
Meek figures a specimen just entering upon the ephebic stage, and 
these coste are faintly indicated. It is, in fact, difficult to see them, and 
they can be felt better than seen in some specimens. The cost are often 
quite linear and distinct on the shell in the neanic stage, but are not present 
before or after this stage. The divaricating ridges described and figured 
by Meek as lines have been described above as lateral chevrons with the 
apices pointing orad and occurring only on the outer thirds of the sides of 
the shell. They are very plain on the inner layers of shell and faintly 
indicated on the cast and entirely independent of the growth bands. At an 
older stage (probably the metephebic substage) than that figured by Meek 
they are quite broken or interrupted by the bands of growth on one side 
where the nacreous layers are preserved, and on the other, in which part of 
the outer layer covers them, they are not visible. They are apparently 
characteristic of the neanic and part of the ephebic stage. The yenter 
retains its flatness until the shell is very large. The sutures are really at 
considerable distances from each other, but the saddles are so deep and the 
lobes so long and narrow that the external outlines are approximated 
except on and near the venter. The first lateral saddles are straight and 
narrow, and there is consequently a band on either side of the venter in 
casts which is not cut up by intermingling sutures. On breaking down a 
specimen sent me by Professor Ward the young at diameter of 11.13 mm. 
from line of involution to venter had the first four saddles even at this 
early stage more slender and more deeply cut by the marginal lobes than in 
the specimens supposed to be young of P. placenta of the west at diameter - 
of 25 mm. The lobes and saddles were also longer and narrower in 
proportion, the sutures nearer together, and the branches of the ventral lobe 
larger and longer and the ventral saddle with larger marginal lobes at 
exactly corresponding ages. ‘The ventral crenulations or tuberculations 
are not so persistent as in placenta of the west, since they disappear in all 
of these specimens in the ephebie stage. 
The incomplete living chamber is about one-half of a volution in | 
length. 
The first volution of a specimen in the collection of the Museum of 
Comparative Zoology, from Clifford, Nebraska, has a depressed rounded 
voniatitie form with a single constriction in this specimen at the end of 
the first quarter. The lateral sutures along the sides have the broad lateral 
