LAMELLIBRANCHIATA. 11 



of the posterior side, and thence concave to the beaks, giving the shell an 

 appearance of considerable amplitude behind. 



Left valve regularly and moderately convex. The right valve is unknown. 



Hinge-line straight, about one-half the length of the shell. 



Beak rounded, extending to and situated near the centre of the hinge-line ; 

 umbonal angle smaller than usual, being about 90°. 



Ears comparatively small, triangular, with concave margins ; posterior ear 

 the larger, separated from the body of the shell by a diiference in convexity 

 and the absence of radiating lines ; anterior ear defined by a sulcus. 



Test marked by about 60 fine, rounded, radiating costa?, with a few inter- 

 mediate smaller ones, and fine stride of growth. The rays are absent on the 

 ears, but the lines of growth are more distinct than on the body of the shell. 



The characters of the interior have not l)een observed. 



The specimen described has a height of 37 mm., and a gi'eatest length of 

 26 mm. 



This species differs from A. formio by its smaller ears, their distinct character, 

 and the general surface-markings. It resembles young individuals of A. 

 princeps ; but the comparative size and extent of the ears are very different, 

 while the body of the shell is more constricted at the base of the ears. A. 

 fasciculatus is very similar in the general form of the body of the shell ; but the 

 length is greater than the height. The alaj and the ornamentation of the test 

 are distinguishing characters. 



Formation and locality. Hamilton group, Schoharie, N. Y. 



AviCULOPECTEN FASCICULATUS. 



PLATE V, FIGS. 9-17 ; AND PLATE LXXXI, FIGS. I-I. 



Aviculopecttn fativUulatus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations; PI. 5, tigs. !t, 10, 



12, 16, 17. Jan.. 1883. 

 Avieulopecten repletits, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 5, tigs. 13-15. Jan., 1883. 

 Orestes, " " " " " PI- 5, fig. 11. 



Shell obliquely and transversely ovate ; height more than seven-eighths of the 

 length, produced behind. Pallial margins regularly rounded, becoming more 



