SPHENODISCID®. 1S) 
locality is probably Fox Hills, Dakota. It is 263 mm. in diameter without 
the shell, which was not present on the venter. The shell would have 
increased the measurement to about 267.5 mm., judging by the shell 
preserved on the venter of the next inner volution. 
The living chamber is complete and about one-half of a volution in 
length, and the diameter, from line of involution to venter, is 150 mm. 
The outlines of the aperture are sufficiently well preserved to show that it 
has in this old stage very low, broad crests on the sides, slight sinuses 
between these and the rostrum; this last being also obtuse and not much 
extended, the outlines being the same as the lines of growth described 
elsewhere. Even at this large size the venter is still acute. The inner 
volution is covered and the umbilicus is only about 1.5 mm. It is as near 
as possible to being completely closed by the thick shell, although in casts 
it is obviously much wider open. The shell is about 3 mm. in thickness 
near the lines of involution. It is also quite thick over the entire last 
volution, whenever present, and consists of the usual outer opaque, middle 
iridescent, and inner pearly layers. The keel was solid, so far as seen, at 
all stages. 
There are fold-like obscure costa, very broad on the oldest part and 
visible only on the outer part of the volution, as in the casts of S. lenticularis 
var. splendens. ‘The venter has the same form as in the young, namely, an 
ax-like solid edge with biconvex sides to the edge, forming obscure smooth 
zones on either side. There are no tubercles on any parts visible, but the 
bands of growth, as in other species, are very well marked on the outer 
shell. The sutures (PI. IX, fig. 10) have broader saddles than those of 
Meek’s figure on pl. 34, but they are quite similar to those of S. lenticu- 
laris variety splendens, except that they are more complex and have a larger 
number of divided saddles. 
The branches of the ventral lobe are similar to those of S. lenticularis, 
but longer and narrower. The first lateral is much shorter than the 
ventral branch and the second lateral lobe, and the second lateral is shorter 
than the third lateral, so that this part of the suture is decidedly convex 
and has not the flat aspect of the same part in Meek’s figure. ‘The lobes 
are also of the trifid type, a difference not accounted for by the difference 
of age, as is the fact also that they are more complicated in outline. The 
fourth to the eighth are quadritid, as are the second, third, and fourth in 
