74 
PALAEONTOLOGY OF NEW-YORK. 
461. 32. ATRYPA PLICATULA. 
Pl. XXIII. Fig. 9 o - ft. 
Atrypa plicatula. Hall, Geol. Rep. 4th District, 1843, pag. 71, fig. 4. 
Shell roundish ovate, plicated, sub-trilobate ; valves nearly equal in old shells ; dorsal valve, 
in young specimens carinate from the beak and depressed towards the base, in older shells with 
a mesial sinus or depression extending nearly to the beak ; ventral valve in young shells almost 
flat, very slightly raised in front, in older shells slightly depressed along the centre near the 
beak, and elevated in a mesial fold towards the base ; plications two in the mesial depression 
and three on the mesial elevation, and from six to eight on each side ; foramen extending to the 
beak ; concentric stride not obvious. 
This shell, in its young state, resembles the Atrypa modesta of the lower strata, but in its 
older condition is very easily distinguished. It differs in the young shell in having the ventral 
valve more flat, and is without the larger plication in the centre which distinguishes the A. 
modesta. The dorsal valve of the latter is also distinguished by having a wider space between 
two stronger plications from the beak to base, while in A. plicatula there is no distinction in 
the plications or spaces between them. Near the base of the dorsal valve, also, there is a 
depression or sinus, which, though inconspicuous in young specimens of A. plicatula , never 
occurs at all in A. modesta. The beak of the dorsal valve in the latter species is also more 
extended beyond the ventral valve, and has the appearance of a narrow cardinal area. 
Fig. 9 a, b. Ventral valve and profile view of a young specimen. 
Fig. 9 c. Profile view of another specimen, in which the valves are still quite unequal. 
Fig. 9 d, e. Front view of a young shell and a full grown individual. 
Fig. 9/, g. Dorsal and ventral valves of a specimen having the ordinary characters of full 
grown individuals. 
Fig. 9 k. A larger individual. 
Position and locality. This species occurs in the calcareous shale at Reynale’s basin, asso¬ 
ciated with A. congesta ; and is rarely found in any other locality. 
462. 33. ATRYPA HEMISPHERICA. 
Pl. XXIII. Fig. 10 a - g. 
atrypa hemispherica. Sil. System, 1839, pag. 639, pl. 20, fig. 7. 
Shell hemispheric or semiorbicular, plicated ; ventral valve flat, nearly orbicular; dorsal 
valve convex ; hinge line extended and nearly straight; surface marked by from eight to 
twelve rounded or subangular simple plications, which are crossed by strong undulating and 
imbricating lamellse. 
This beautiful little species is very abundant in this group, occurring principally in the upper 
green shale, where it often forms thin calcareous layers in which the shells preserve a beautiful 
