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PALEONTOLOGY OF NEW YORK. 
length. Their apices scarcely reach the hinge-line and are gently incurved. 
Toward the hinge their slope is abrupt, leaving the hinge-line slightly 
depressed. The posterior slope sometimes shows a low sinus as in many 
species of lamellibranchiata. 
Hinge-line long and straight, extending the entire length of the carapace 
and showing no articulating teeth or ridges. 
The ventral margin is gently recurved and scarcely thickened. 
Surface ornamented by continuous, concentric, sharp ridges, which are 
closely crowded near the apices and more distant on the posterior portions of 
the valves. Of these ridges, twenty-five may be counted in a full-grown 
individual. 
Bimensions. Considerable variation in size is noticeable in the specimens 
at hand. A mature individual has a length of 15 mm., and a width of 13 
mm.; the youngest example a length of 4.5 mm., and a width of 4 mm. 
Observations. This unique species in outline and sculpture presents a striking 
similarity to species of the genus Aptychopsis. It lacks, however, any evidence 
of a rostral plate and of any anterior cleft for the reception of such a plate. 
Its affinities are evidently with the genus Estheria and Leaia, as is shown by 
its umbonate valves, but it differs from the former in its remarkably elongate 
hinge-line, absence of the characteristic surface ornamentation, and from the 
latter in outline, and in the absence of the posterior ridge. It may, therefore, 
be regarded as constituting a new generic type of Phyllopod Crustacea. 
Distribution. Hamilton group. A number of specimens of the species have 
been obtained, all from a single locality, in the limestone at the base of the 
Hamilton shales near Centerfield, Ontario county. 
