PLACENTICERATID ®. 231 
rather uncertain indications of tubercles that may have existed on the outer 
shell, although the inner line on the umbilical shoulders is plainly shown 
on the nacre of the last volution. These last are obviously confined to this 
volution of the ephebic stage, since they are not present on the inner 
volutions, which are covered by the thick, opaque outer layer of the shell. 
This settles the fact that these tubercles may appear very late in the life of 
this species. They are small and wide apart on this specimen when the 
volution from line of involution to the venter is 60 mm., and are not 
present on the external shell in that part of the same volution which is 
about a centimeter apicad across a break in the fossil. There are fine 
transverse wrinkles on the nacreous layer in the ventral zone of this shell. 
A large, much crushed fossil from Loup Fork, Nebr., also collected 
by Dr. Sternberg, is 215 mm. in diameter and has the external shell well 
preserved and as usual very thick, especially on the umbilical shoulders. 
Faint tubercles are present on the last volution and are also preserved on a 
remnant of shell belonging to a volution which has been broken away. 
Such small tubercles as these might be present on a shell and yet be absent 
on a cast. 
A fine specimen in the National Museum, with nacreous layer, from 
southern Colorado, and having typical sutures of the western form and also 
identical in other respects, has a diameter of 136 mm. and shows faint 
tubercles on the nacreous layer. The volution is 65 mm. in diameter from 
line of involution to venter, when the first tubercle appears. This 
specimen makes ft possible to say definitely that these appear in the 
ephebic stage, probably metephebic substage. 
One specimen, from Black Hills region, in the collection of the Boston 
Society of Natural History, shows faint tubercles on umbilical shoulders 
when the diameter is less than 100 mm., and there are faint but perfectly 
defined minute tubercles on either side of the venter. Another specimen in 
the same collection and from the same locality has a diameter of 305 mm. 
It is without living chamber and has the nacreous layer in part preserved. 
There are faint tubercles on the umbilical shoulders of the next inner and 
part of the outer volution. The sides have three obscure broad longi- 
tudinal folds or ridges in the gerontic stage. There are no tubercles on 
the edges of the ventral zone. This zone continues to be very faintly 
concave, even on the last part of the outer volution. -The gerontic stage 
