168 PALEOZOIC PALEONTOLOGY. 
The approximate dimensions of an imperfect right valve from New. 
Jersey are: length, 40 mm.; height, 35 mm.; convexity, 11 mm. 
Remarks.—A large species of Cyrtodonta from the lower portion of 
the Trenton lmestone in New Jersey seems to agree closely with 
Billings’ description and illustrations of C. canadensis, the only dif- 
ference noticeable being that the anterior margin is more broadly 
rounded. None of the species are perfectly preserved, and there may 
be some doubt as to the correctness of the identification, but more 
perfectly-preserved specimens are essential before a specific distine- 
tion can certainly be made. 
WHITELLA SUBORBICULARIS Nn. sp. 
Plate Xe) Bis.) 2: 
Description.—Shell of medium size, erect, ventricose, suborbicular 
in outhne, the hinge-line rather long, arcuate. Beaks of moderate 
size, prominent, strongly incurved; umbones prominent, umbonal ridge 
sharply angular near the beak, becoming more broadly rounded after 
passing the umbonal region. ‘The post-umbonal slope abrupt, concave, 
with the suggestion of a faint second ridge, diverging from near the 
beak, about one-third of the distance from the umbonal ridge to the 
posterior margin. The anterior slope, from the umbonal ridge, is 
slightly convex and less abrupt than the posterior slope. 
The dimensions of a left valve are: length, 22 mm.; height, 24 
mm.; convexity, 10 mm. 
Remarks.—This species is established upon a nearly-complete left 
valve. It may be distinguished from other members of the genus 
by its more erect attitude and its suborbicular outline. In the erect 
attitude of the shell it resembles Plethocardia umbonata Ulr., and it 
is possible that it should be referred to the genus Plethocardia rather 
than to Whitella. The distinguishing character between these two 
genera is a strong process, which projects downward from beneath the 
beak in each valve of Plethocardia, but the New Jersey specimen is 
not in a proper condition of preservation to determine the presence 
or absence of such a process. 
