PLACENTICERATID®. 225 
minute inflections of the outlines. The saddles are not so long as to inter- 
fere with deciphering the outlines of the lobes until the fourth volution is 
reached and the anephebie substage begins. Before this the sutures 
resemble more those of P. bolli, having shorter saddles and probably at still 
earlier stages they are even more like those of this species, being propor- 
tionately shorter and with simpler digitations. There are nine lobes present 
on the last quarter of the third volution. The three principal lobes have 
their usual proportions, and the ventral iobe is nearly the same as in the 
adult, but the siphonal saddle is not so prominent, and the minor saddles 
on the sides of this are also much smaller and more nearly of the same size. 
There are six lobes on the lateral zone, a seventh on the shoulder, and two 
on the umbilical zone. The saddles are all distinctly bifid, except the tenth, 
which is not yet differentiated. The lobes are all of the trifurcate type, 
except the ninth, which is not fully developed and is single or unsym- 
metrical. The lobes and saddles greatly increase in complication of outline 
and become larger and larger, but the number remains stationary on the 
fifth volution. Meek figures a very large suture with twelve lobes. 
~ Tubereular elevations make their appearance on the edges of the 
ventral zone in the neanic stage, but they are more perceptible to the touch 
than to the eye. The widely separated sigmoidal coste are more distinct, 
but the deep apical bend is only one-half developed and ends abruptly in 
some with a faint tubercle. The ventral part of the bend is apparently 
absent on the fourth volution, but subsequently appears more decidedly on 
the last quarter of this volution. Internally the oral bend of these costa 
is also deficient in the neanic stage, appearing to be better developed in 
an ephebic substage. Nevertheless, when one looks at the volution, he is 
apt to see only the inner half of the deeper apical bend. The chevron-like 
folds are present on the shell in the later neanic substages and may come 
in earlier. A line of very faint, hardly perceptible, tubercles appears on 
the umbilical shoulder on the fifth volution in an ephebie substage. 
~ [have been as minute in my descriptions as the specimen in hand 
permitted, because the presence of these indistinct tubercles and costz in 
the neanic and early ephebic substages show, together with the more widely 
separated sutures and broader venter, that the young are quite similar to 
those of placenta and have also traces of their affinity with the more heavily 
@Mon. U. S. Geol. Sury. Terr., Vol. LX, p. 466. 
15 
MON XLIV—03 
