LAMELLIBRANCHIATA. 1 1 



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

 appearance of considerable amplitude behind. 



Left valve regularly and moderately convex. The righl 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 binge-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 difference in convexity 

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



Test marked by about 60 line, rounded, radiating cost®, with a few inter- 

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

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



The characters of the interior have not been observed. 



The specimen described has a height of 37 mm., and a greatest length of 

 l!(i mm. 



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

 and the general surface-markings. It resembles young individuals of .1. 



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. 

 fasckulatus is very similar in the general form of the body of the shell : hut the 

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

 are distinguishing characters. 



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



AviCULOI'ECTEN FASCK'ULATUS. 

 PLATE v. PIGS, B 17; isn 1M.ATK I.\\\l. PIGS. I I. 



Ar'n ulopecten fa . ilvit.. Pal. N. V.. vol. v. pt. 1. Plates and Explanations: PL .".. figs. 9, 10, 



P-\ Hi. 17. Jan.. 1888 

 Aeiculopectenrepletu8,llA£L. Pal. N. V.. veil. v. pt. 1. Plates and Explanations : PI. 6, figs. 18 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 



