-us 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. 



Dimensions. 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 

 binge-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. 



