268 



PALjEONTOLOGY of NEW-YORK. 



627. 41. ATRYPA NITIDA. 



PL. LV. Fig. la- 



jitrypa nitida. Hall, Geol. Rep. 4th Dist. N. York, 1843 : Tables of Organic Remains of 

 Niagara Group, No. 14, fig. 5. 



Shell ovoid, with the beaks more or less extended ; surface smooth, or with fine concentric 

 striae and a few conspicuous lines of growth towards the base, and sometimes on the middle of 

 the shell ; valves nearly equally convex, the beak of the dorsal valve being much elevated 

 above, and incurving over the ventral valve ; the dorsal valve sometimes marked, near the base, 

 by a longitudinal depression. 



This is a very abundant species in the shale at Lockport, where it presents considerable 

 variety of form and proportions. It is usually remarkable for its smooth surface, interrupted 

 only by a few lines of growth on the middle or towards the base, the finer concentric striae 

 being obsolete or invisible to the naked eye. In some individuals the shell becomes thickened 

 near the base, and marked by very strong lines of growth. 



We find no difficulty in tracing the characters of the species from the most minute specimen 

 figured to the largest (fig. 1 /) ; beyond which it is difficult to find specimens showing a grada- 

 tion to the larger individuals of fig. 2 and 3, which are closely allied forms, and perhaps only 

 varieties of the same species. 



Fig. 1 a, b, c. Ventral views of three individuals of small size. 



Fig. 1 d. Profile view of fig. 1 c. 



Fig. 1 e. A gibbous specimen, where the outline is narrow in proportion to the length. 



Fig. 1 /. Profile view of the same specimen. 



Fig. 1 g. Front view of the same. 



Fig. 1 h, 7, k. Ventral, profile and front views of a specimen, where the valves are very much 

 thickened towards the bases. 



Fig. 1 I. A large individual of the same species. 



Fig. 1 m, n. Profile and cardinal views of other specimens. 



Fig. 1 0. Ventral view of a large specimen, which is inequilateral from pressure. This feature is 

 very common ; and the specimens being usually compressed, and often elongated, 

 give the impression that there is a distinct species presenting these characters. It Avill 

 be found, however, that this is only an accidental character, and often occurs in the 

 shorter and more gibbous specimens. 



Position arid locality. This species occurs in every part of the shale of the Niagara group, 

 and in some localities is quite abundant. The most prolific locality is at Lockport, but it is of 

 common occurrence at all the exposures of the shale as far eastward as Rochester, and is found 

 at Wolcott in Wayne county. 



