286 PALEONTOLOGY OF NEW-YORK. 
deviation than is often observed in the same shell in the limestone. The surface is marked 
by concentric lamellose striz, and the centre by nearly equal longitudinal strie ; the sides 
are more or less straight, the base rounded with the upper extremity often subcuneate, 
having the slopes nearly direct. The base is sometimes nearly straight, and the shell 
resembles L. lewis. 
Figs. 1 a, b. Figures of two specimens, showing the difference ordinarily seen in the form of this fossil. 
Fig. 1c. A portion of the surface magnified. 
Position and locality. In the soft argillaceous shales in the lower part of the group at 
Loraine, Turin, and other places. (State Collection.) 
141. 4, LEPTASNA ALTERNATA. 
Pui. LXXIX. Figs 2a-1. 
Reference pag. 102, pl. 31 and 31 a, of this volume. 
Strophomena nasuta. Conranv, Jour. Acad. Nat. Sciences, Vol. viii, p. 160. 
— — Emmons, Geol. Report, 1842, pag. 403, fig. 3. 
Strophomena. Ip. Ib. pag. 4038, fig. 2. 
This species occurs under a variety of conditions, and often presents a different aspect 
from the same in the Trenton limestone ; but a comparison of several hundred specimens 
has convinced me that it is identical. The casts of the interior are striato-punctate, as in all 
the Lerprana ; and the alternating character of the striz is not well preserved, or even at 
all visible. The circumstance of having one or two of the central lines larger than the 
others is often observed, and we have the same, already noticed, in those of the limestone. 
This character, therefore, which has been regarded as reliable for distinguishing it from 
L. alternata and L. deltoidea, is of no importance. It presents in these rocks, as in the 
limestone below, all the varieties from forms acutely nasute in front, to those of regularly 
curved outline, and entire absence of this character. 
The impression of the outer surface of the shell, so often preserved in these rocks, shows, 
in a very perfect manner, the beautiful fascicles of striae, separated by larger ones. This 
character, however, is not always evident; neither does it exist equally in all specimens. 
In a few localities we find this species with the shell perfect, showing in all its variety the 
same character which the species every where exhibits, and which is even more strongly 
marked and better preserved than is usual in the limestone. The most interesting parts of 
the shell, however, are the moulds of the interior of the valves, which are never found in 
the Trenton limestone. These reveal important characters of the species, on which we 
may rely in distinguishing it from others. 
Fig. 2a. Mould of the interior of the convex valve of this species, showing the form of the visceral 
impression, the oblique teeth, etc. The nasute character is well preserved. 
Fig. 2 b. Figure of a similar specimen, where the nasute character is less conspicuous. 
Fig. 2c. A smaller specimen of the same character, showing a slight contraction in front. 
The entire surface in such specimens, when well preserved, is striato-punctate, 
