312 PALEOZOIC PALEONTOLOGY. 
Remarks.—The internal characters of this species have not been 
observed, but it is apparently a member of the family Centronellida, 
although it may not belong to the genus Centronella. In some re- 
spects it resembles members of the genus Oriskania, especially in the 
general outline of the shell, the infolded cardinal margin of the 
pedicle valve and the pointed beak, and this species may be an early 
representative of that genus. 
ATRYPINA IMBRICATA (Hall). 
Plate XXXVIL., Figs. 17-22. 
1859. Leptocelia imbricata Hall, Pal. N. Y., vol. IIL., p. 246, pl. 38, 
figs. 8-13. 
Description.—Shell small, varying from subcircular to subrhom- 
boidal and to longitudinally subsemi-elliptical in outline; hinge-line 
shorter than the greatest width, cardinal angles rounded. Pedicle 
valve convex, most prominent at about the middle, sloping to the 
lateral and cardinal margins; beak small, pointed, shghtly incurved, 
perforated at its apex by the foramen. Brachial valve depressed-convex 
or nearly flat, its beak scarcely produced beyond the hinge-line. Sur- 
face of each valve marked by from six to twelve strong, rounded, radi- 
ating plications, of which some of the central ones sometimes divide in 
the larger specimens. On the pedicle valve the two central plications 
are larger and more elevated than the others, the depression between 
them forming a narrow mesial sinus, while in the brachial valve there 
is one larger median plication opposite the median depression of the 
pedicle valve. The radiating plications are crossed by strong, imbri- 
cating, lamellose, concentric lines of growth. 
The dimensions of a large specimen are: length, 9 mm.; width, 
9.5 mm.; thickness, 4.5 mm. 
Remarks.—This species has been observed only in the shaley mem- 
ber of the New Scotland beds, where it is not usually abundant. It 
occurs almost universally in the form of moulds of the exterior and 
casts of the interior, the brachial valve frequently being so crushed 
as to appear to be strongly concave. 
