LAMELLIBRANCHIATA. 19 



Interior characters not known. 



The original of this species is 20 mm. in height, 19 mm. in longitudinal 

 diameter, and the length of hinge-line 12 mm. Another specimen is 14 

 mm. in height and 12 mm. in length. 



This species is distinguished from A. duplicatus and A. rugastriatus by the 

 less rugose surface, smaller size and different outline. The form of the ears 

 and byssal sinus is very similar to A. duplicatus, but differs in the regular 

 reticulate surface ; that species is also a comparatively longer shell. It is in 

 shape more like A. rugastriatus. 



Formation and localities. In the middle members of the Chemung group, 

 at Philipsburgh and Rockville, Alleghany county; Conewango, Cattaraugus 

 county ; and near Westfield, Chautauqua county, N. Y. 



AviCULOPECTEN ^QUILATERALIS, n. Sp. 

 PLATE LXXXII, FIG. 1. 



Shell of medium size, transversely ovate, erect, equilateral ; height equal to 

 the length ; lateral and basal margins regularly rounded ; upper anterior 

 and posterior margins extending in nearly a straight line to the beak. 



Left valve slightly convex ; greatest convexity on the umbo. 



Hinge-line straight, nearly tw.o-thirds the length of the shell, situated 

 centrally. 



Beak well-defined, situated near the centre of the hinge-line. 



Ears sub-equal, triangular, defined from the umbo by a sinus in the margin, 

 with a sulcus extending to the beak ; margins slightly convex ; extremities 

 rounded. 



Test ornamented with al)out sixty regular, straight, alternating rays, 

 crossed by fine, irregular strias of growth. The rays are present on the ears, 

 and are more numerous on the anterior ear. 



The specimen described has a length and height of about 38 mm., and 

 hinge-line of 22 mm. 



