PLACENTICERATID. 227 
on the line of involution. Six were on the lateral zone, one on the sharply 
defined umbilical shoulder, and three on the umbilical zone. The last sad- 
dle showed a distinct marginal lobe, the next saddle single, the succeeding 
saddles were also bifid and had entire outlines, the fourth had begun to 
show marginal digitations,.and the remaining saddles were more or less 
deeply cut, having approximately the forms of the ephebic stage, but being, 
of course, much simpler. The outlines of all of them were free except 
those of third and fourth saddles and those of the third lateral lobe. The 
umbilical lobe is probably entire, as is the next lobe; the eighth is 
symmetrically trifid, the next from its position on the umbilical shoulder is 
unsymmetrically trifid, the next lobe on the side is symmetrically trifid 
again. The remaining lobes show ephebic division already defined but 
simpler than in the adult, and the same is true also of the ventral 
lobe and saddle, and the bare spaces on the cast on either side of the 
venter would be as conspicuous as in the later stages if the sutures 
were as close together. One thing is noticeable in this specimen; the 
slightly younger sutures on the same volution are for a time slightly closer 
than the succeeding ones, owing to a temporary decrease in the rate of 
growth of the shell. -This specimen had sutures quite different from the 
sutures of the small specimen above described, in which at the same age 
there were approximating and even decidedly overlapping outlines, as in 
the adult. The outiines themselves, however, were about the same in both 
specimens, so that the differences were merely those of the slower, less 
vigorous growth of the forms as compared with that now being described. 
At the beginning of this volution, when the diameter from lines of 
involution to venter is 7 mm., the umbilical zone is just beginning to be 
formed. The ventral saddle at this time is just beginning to show 
digitations on its sides, and is broad and large with flat concavity across 
the venter. The first lateral saddle is distinctly trifid, the second and third 
laterals with club-shaped bases and almost entire, showing only the faintest 
possible trace of the median marginal lobe that divides them in the 
succeeding sutures; the fourth lateral has this marginal lobe more distinct, 
but still very small, and the remaining saddles are entire with somewhat 
flattened basal lines. 
The arms of the ventral lobe and the tops of the first and second and 
third laterals are unsymmetrically trifid, the fourth lateral is just beginning 
