TRENTON LIMESTONE. 141 



179. 12. ATRYPA EXIGUA (n. fpi). 



Pi.. XXXIII. Figs. 6 a, b, c, d. 



Plano-convex ; length and breadth about equal ; cardinal line considerably extended ; 

 dorsal valve elevated in a ridge along the middle, depressed at the sides, and slightly 

 inflected towards the cardinal extremities ; beak small, straight, much extended beyond 

 the cardinal line ; ventral valve considerably shorter than the dorsal, depressed-convex, 

 with a broad depression along the centre, reaching half way from the base to the beak ; 

 beak small, and close pressed into the foramen beneath the beak of the opposite valve ; 

 surface scarcely marked with fine concentric lines, and a few indistinct longitudinal rays 

 near the margin. 



In the largest specimen which I have seen, there are evidences, under a magnifier, of 

 small radii commencing below the centre of the valve. Since, however, they are not 

 perceptible to the naked eye, they are of minor importance, unless it should be found that 

 this is the young of a species which changes with growth. The specimens yet seen, how- 

 ever, are minute, and it may properly be doubted whether the species attains a size beyond 

 the largest figures given. The valves are often close pressed, and deflected at the margin. 



It bears considerable resemblance to a small species occurring in the. Schoharie grit; but 

 that one is readily distinguished by the absence of any depression in the ventral valve, 

 which is a rare feature among the Atkyp.s:, and probably indicates a departure from J.he 

 true normal character of the genus. 



Fig. 6 a. Ventral valve : the depression is proportionally too narrow in the figure. 

 Fig. 6 b. Dorsal valve of the same. 



Fig. 6 c. Cardinal view, showing the acute slightly incurved beak. 



Fig. 6 d. Two figures of the dorsal valve of a small and large individual, the form and extension of the 

 acute beak being well represented. 



Position and locality. In the central part of the Trenton limestone at Lowville, and near 

 Martinsburgh, Lewis county. (State Collection.) 



180. 13. ATRYPA MODESTA. 



Pl. XXXIII. Fig. 15. 

 Prodruta modetla. Sat, Collection of the Acad. Nat. Sciences, Philadelphia, 



Suborbicular or plano-convex, with the beak extended ; width a little greater than the 

 length ; cardinal line distinctly marked and somewhat extended ; dorsal valve convex, 

 with an elevated ridge along the centre, occupied by four plaits which are stronger than 

 the others ; beak prominent, incurved and perforated, the perforation extending below the 

 l>eak and occupying a portion of the area ; ventral valve depressed-convex, broadly oval 



t 



