LAMELLIBRANCHIATA. 75 



of the posterior ears, do not assimilate it with the ordinary characters ascribed 

 to those genera. 



Formation and locality. In the Clieniung group near Elmira, N. Y. 



Pterinopecten imbecilis. 



PLATE XXIV, FIG. 1. 



Pleriiiopecteti, imherillt. Hall. Pal. N. Y., vol. v, pt. 1. Plate.s anil Exiilanatidiis : PI. 24, tig. 1. 



Jan.. 1S83. 



Shell small, rhomboidal, oblique, much longer than high ; anterior and basal 

 margins rounded ; posterior margin produced. 



Left valve moderately convex. Right valve not known. 



Hinge-line equal to, or greater than, the length of the body of the shell. 



Beak obtuse, directed forward, rather prominent. Umbonal region ample, 

 defined anteriorly ; subtending an angle of about 90°. 



Ears very unequal, triangular. Anterior ear small, defined by a distinct 

 sulcus. Posterior ear large, undefined ; margin concave ; extremity angular. 



Surface marked by numerous strong, simple radii, with wider interspaces 

 which have usually finer intermediate radii ; crossed by fine, equal concentric 

 lines of growth, and stronger, more distant lamellas, which are elevated and 

 arched backwards over the larger rays, producing nodes. 



Internal characters not preserved. 



The specimen has a length of 15 mm., height 11 mm., hinge-line about 

 15 mm. 



This species differs from P. latus, by its distinct and widely separated rays, 

 and more oblique outline. 



Formation and locality. In the Chemung group at Tioga, Tioga county, Penn- 

 sylvania. 



