54 
Allodesma ? umbonata n. sp. 
Plate IV, figure 4 
Shell small, exact measurements not available, as there is 
no complete specimen; the contour, however, indicates a small 
shell, the largest specimen being approximately 14 mm. long and 
8 mm. wide. The other two fragments when completed would 
apparently be about two- thirds of that size. Anterior margin 
rounded, posterior margin not preserved. Umbonal ridge 
prominent, somewhat flattened on the top, ending anteriorly in 
a small prominent beak, only slightly incurved. Towards the 
posterior the umbonal ridge gradually disappears in the generally 
moderate convexity of the shell. A shallow sinus crosses the 
shell obliquely, broadening out and almost disappearing towards 
the ventral margin. Concentric growth lines are present and 
are more prominent towards the margin. On one of the moulds 
is the impression of two obscure rays on the posterior umbonal 
slope. 
Anterior muscle scar very marked, bounded on the top and 
posterior side by a deep groove, extending downward from the 
hinge in front of the beak. There is no trace of the posterior 
scar, but, if present on the somewhat imperfect posterior, it must 
be very faint. The pallial line is not apparent on the specimens 
found. 
One specimen exhibits a piece of a broken cardinal tooth; 
the rest of the tooth-bearing portion is crushed and mixed with 
the matrix. 
The placing of this species in the genus AUodesma is not 
quite satisfactory, and cannot be so until a specimen is found 
showing the tooth system more clearly. Also, the one species 
of that genus already described comes from the Trenton forma- 
tion. The Chazy species differs from Allodesma ellipticum in 
having a more obtusely-rounded anterior margin, and a more 
prominent umbonal ridge which results in producing the shallow 
sinus anterior to the ridge. The concentric growth lines and 
the obscure radial lines are coarser than in the Trenton species. 
Also,- the groove marking the ridge at the posterior border of the 
anterior muscle scar is curved, not linear. 
