SPHENODISCID ®. 61 
The branches of the ventral lobe are trifid and spreading as usual, but 
small, of course, in such a suture; the first laterals are shorter and broader 
and together with the longer second and third laterals form the usual arch. 
The fourth lateral is about half the length of the third and from this to the 
umbilicus there is the usual diminishing row. The type is bifid except the 
first and fourth laterals; these were uncertain. All are divided except, 
perhaps, the last and umbilical lobes; these were not seen. 
There are twelve saddles and twelve lobes visible, and there may be a 
saddle on the line of involution on the left side; the sutures of the right 
side were not seen. 
The older specimen of this species (PI. ITI, fig. 13) is a cast from same 
locality and is very instructive. It is 90 mm. in diameter and the form 1s 
better preserved, not having been injured by pressure. The oldest part is 
47 mm. in diameter, the umbilicus without the shell is about 8 mm., and the 
volution opposite is 85 mm. measuring from line of involution to venter. 
The greatest transverse diameter is carried farther out than the centran 
surface at this age and is 21 mm. between the tubercles, the greatest 
diameter opposite between tubercles and about centran of the lateral zones 
is 16 mm. No shell is present in any of these measurements. 
The shell is excessively thick on the umbilical zones and is nearly a 
millimeter thick on the sides in the parephebic substage; it is considerably 
thicker on the venter, where it forms a solid keel. There are two lines of 
tubercles as in the smaller specimen, the age of the first part of this volu- 
tion being the same as the age of the last part of exposed volution in the 
younger specimen, but the nodes are rounder or hardly perceptibly elon- 
gated. The inner line of nodes persists and retains its distance from the 
umbilicus, but the nodes are slightly nearer to the venter on the last part 
because of the gerontic decrease in the rate of growth of the dorsoventral 
diameter. The outer tubercles gradually decrease and disappear, but the 
fold-like short costee between the tubercles persist. These costae may have 
a bifurcated aspect when slightly better developed than in these specimens. 
The outer part of the whorl is decidedly convex, while the surface between 
the inner line and the umbilical shoulder is decidedly concave and the 
umbilical zone narrow and abrupt. 
The earliest part of this volution has the venter bluntly acute, show- 
ing it to be in the parephebic substage, while on the second quarter the 
rounding of the venter and loss of tubercles shows anagerontic substage. 
