268 
PALAEONTOLOGY OF NEW-YORK. 
627. 41. ATRYPA NITIDA. 
Pl. L,V. Fig. 1 a - o. 
Atrypa 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 strise 
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 l) ; 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, i, k. Ventral, profile and front views of a specimen, where the valves are very much 
thickened towards the bases. 
Fig. 1 l. A large individual of the same species. 
Fig. 1 m, n. Profile and cardinal views of other specimens. 
Fig. 1 o. 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 will 
be found, however, that this is only an accidental character, and often occurs in the 
shorter and more gibbous specimens. 
Position and 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. 
