166 PALEOZOIC PALEONTOLOGY. 
a tendency to become angular just beneath the middle and obliquely 
subtruncate above; basal and dorsal margins broadly convex. Beaks 
small, somewhat tumid, placed about one-third of the length of the 
shell behind the anterior extremity. Surface marked by fine, con- 
centric lines and several stronger varices of growth; the latter show 
through the shell, so as to be visible on casts of the interior. Hinge- 
plate narrow, not over half the length of the shell, minutely toothed ; 
denticles twenty:or more in each valve, three-fourths of the number 
being posterior to the beaks, placed obliquely and so that they con- 
verge inwardly, the direction of the anterior series being nearly at 
right angles to that of the posterior series. Clavicle strong, nearly 
straight, almost vertical, sharply defining the somewhat semi-circular 
and large anterior, muscular scar, and leaving a strong furrow in casts 
of the interior just in advance of each beak. The furrow extends 
beyond the middle of the distance to the basal margin. Posterior 
scar faint, smaller than the anterior, occupying a central position on 
the post-cardinal slope. Several small umbonal scars may be observed 
on good casts, and obscure rays are occasionally visible on their 
sides.”—U rich. 
The dimensions of a left valve are: length, 9 mm.; height, 5 mm. 
Remarks.—Only a few specimens of this species have been found 
in the Trenton limestone of New Jersey. They are all more or less” 
imperfect casts of the interior of the shell, and are somewhat more 
pointed posteriorly than the typical members of the species. If a 
sufficient number of properly-preserved specimens were available for 
study, it is possible that the New Jersey shells would be found to 
constitute a distinct species, but for the present it seems best not to 
attempt to separate them. 
CYRTODONTA BILLINGSI Ultr. 
Plate Xs  Migsei: 
189%. Cyrtodonta billingsi Ulr., Pal. Minn., pt. IL., p. 538, pl. 40, 
figs. 2-6. 
Description.— “Shell of medium size or less, transverse, obliquely 
ovate, highest in the posterior half; valves strongly ventricose in the 
umbonal and central regions. Hinge-line at least two-thirds the length 
of the shell, slightly arcuate, posteriorly declining and passing grad- 
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