PLACENTICERATID ®. 219 
One specimen, a fragment given by Dr. R. 'T. Jackson, said to have 
‘come from Bad Lands, near Black Hills, South Dakota, is 73 mm. from the 
lines of involution to the venter at small end, and 88 mm. at a distance of 
110 mm., measured along the centran axis of the side, or somewhat more 
than one-fourth of a volution farther on, the whole length being 160 mm. 
This fragment is a living chamber, the last septum and the aperture being 
partly preserved. A restoration of the whole coil shows the diameter of the 
entire specimen to have been about 174mm. The obsolescing cost, sparse 
and small tubercles on umbilical shoulders, and small, more closely set 
tubercles on the edges of the venter, show that this is probably the 
parephebic stage of this species. 
The venter has become rounded, or rather the previously concave zone 
has become convex, but the ventral tubercles, although faint, are clearly 
discernible at the oldest end of this fragment. 
Thus the gerontic stage must have begun in this species at a size when 
the ephebic stage was not yet completed in P. whitfieldi or placenta. The 
saddles of the last septum show much less complex outlines than in whitfieldi. 
The third lateral lobe was not entirely preserved, but it was obviously not 
so long.. The outlines resemble approximately those of boll. Dissecting 
out a part of the ephebic volution contained in the zone of involution, the 
shell and the sutures also were found perfectly preserved. The probable 
diameter of the volution at this age from line of involution across side to 
venter was about 35 mm., and the whole diameter of coil perhaps 75 mm. 
The shell at the younger end of this fragment, which was about one-fourth 
of a volution, showed a decidedly concave venter; the cast was also slightly 
concave. At the other end, while the shell was still concave, the cast was 
flat on the venter. The tubercles on the younger end were well defined, 
but mere fine crenulations, asin P. placenta and whitfieldi. They were barely 
perceptible on the cast at this end of the fragment, and not visible at all on 
the cast at the other end, although, as stated above, present on the cast of 
the gerontic living chamber. The sutures showed somewhat more solid 
branches on the saddles than in whitfieldi, and ventral lobes and siphonal 
saddles like those of P. intercalare. Previous to cracking out this fragment 
the specimen was classified with variety bolli. The chevron markings on the 
nacreous layer were beautifully displayed and very instructive. At the 
younger end they had the normal orad direction for about an inch, then 
