164 PALEOZOIC PALEONTOLOGY. 
about one-third of the distance to the posterior extremity. Surface 
marked by obscure, concentric lines.”—UlIrich. 
The dimensions of a large right valve are: length, 58 mm.; height, 
26 mm.; convexity, 8 mm. 
Remarks.—Two forms of shells in the Trenton limestone of New 
Jersey have been identified as C. nasuta. One of these is represented 
by the large specimen which has been illustrated, and occurs in the 
lower portion of the formation, associated with Leperditia fabulites, 
&c., in strata which are doubtless of Black River age. All of the speci- 
mens. found at this horizon are more or less fragmentary, but all are 
of this large form of the species. Much higher up in the formation, 
associated with a typical Trenton fauna, a small form of the species 
is occasionally met with, which does not have a length to exceed 25 
mm. So far as the specimens of both forms are preserved, they seem 
to be identical in all respects, except in size. 
CTENODONTA JERSEYENSIS Nn. sp. 
Plate Xa., Hig. 17. 
Description.—Shell of medium size, moderately convex, longitudi- 
nally subovate in outline. Beak small, situated a httle in front of 
the middle, closely incurved and pointing posteriorly. Umbo flat- 
tened, umbonal ridge rather prominent, the posterior slope being 
abrupt and slightly concave; in front of the umbonal ridge the sur- 
face is flattened and very gently sloping towards the anterior ex- 
tremity of the shell to near the margin, where it drops off abruptly; 
the slope from the umbo to the basal margin is gently concave. Ex- 
tending from the beak across the umbo, directly downward towards 
the basal margin, is a shallow, indistinct sinus, which becomes obso- 
lete below the middle of the shell. Surface marked by inconspicuous, 
concentric lines of growth. 
The dimensions of a well-preserved right valve are: length, 12.5 
mm.; height, 10 mm.; convexity, 3 mm. 
Remarks.—This species has been found only in the very base of the 
Trenton formation. In its flattened surface, extending from the 
umbo towards the basal margin, this species resembles C. carinata 
Ulr., but the two species are quite different im outline, and in C. jer- 
seyensts the greatest convexity is on the posterior umbonal slope rather 
than in front. The New Jersey shell is also the larger. 
