142 PALEONTOLOGY OF NEW-YORK. 
or nearly circular, with a broad but ill defined sinus along the middle, the central plication 
stronger than the others, with a smaller one on each side ; each valve with about 18 simple 
rounded plications ; surface obscurely punctate. 
This neat and beautiful little species well merits the name bestowed upon it by Mr. Say, 
which I am happy to preserve. It is a rare species in New-York, while it is abundant in 
Ohio, Indiana and Kentucky. It is, in a large number of specimens examined, very con- 
stant in the characters given above. The central elevated portion consists of four stronger 
plications, which appear to be in two pairs, from being separated in the middle by a deeper 
and wider groove, while the groove between each pair is narrower than between the other 
plaits of the shell. These characters, with the stronger plication in the centre of the ventral 
valve, are constant, and apparently reliable in distinguishing the shell. The mesial sinus 
on the ventral valve, which is scarcely conspicuous in young shells, becomes deeper and 
more strongly marked in older ones, and produces a strong depression in the front of the 
shell. 
Fig. 15, The two upper figures are of the dorsal and ventral valve of a specimen above the common 
size. The lower figure is of a small specimen, having the same characters. 
Position and locality. I have seen this species in New-York only from the Utica slate, or 
upper shaly part of the Trenton limestone, at Turin, Lewis county. It is quite abundant at 
numerous western localities, particularly Oxford and Cincinnati (Ohio), Madison (Indiana), 
Frankfort and Maysville (Kentucky) . 
This species, with the three preceding ones, form a group, presenting characters which 
may require a separation from the true Arrypm. These characters consist in the elevation 
of the dorsal valve along the centre, with a depression or sinus on the ventral valve, being 
the reverse of the usual arrangement. The beak is incurved, with a perforation at the apex, 
which occupies, also, a part or all of the deltidial area, being usually narrow and long. 
Mr. Conrap, some time since, proposed the name Stenocisma for some specimens of the 
group of Arrypm or TEREBRATUL#, which he subsequently abandoned. Should the charac- 
ters here noticed be found persistent, and accompanied by the narrow foramen, I propose 
to restore the name first indicated by Mr. Conrap for the genus. 
181. 14. ATRYPA CIRCULUS (n. sp.). 
PL. XXXIII. Figs. 7 a, b,c. 
Depressed orbicular ; valves equally and uniformly convex ; margins close pressed, with 
scarcely a conspicuous undulation in front ; beaks equally prominent, one slightly larger, 
approximate ; surface marked by fine concentric lines, and obsolete longitudinal striz, 
which are only visible by the aid of a magnifier. 
