PALiEONTOLOGY OF NEW-YORK. 



deviation than is often observed in the same shell in the limestone. The surface is marked 

 by concentric lamellose striae, and the centre by nearly equal longitudinal striae ; 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. leioisii. 



Figs. I a, b. Figures of two specimens, showing the difference ordinarily seen in the form of this fossil. 

 Fig. 1 e. 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. LEPTiENA ALTERNATA. 



Pu LXXIX. Figs 2 a - /. 

 Reference pag. 102, pi. 31 and 31 a, of this volume. 

 Slrophomena nantta. Conrad, Jour. Acad. Nat. Sciences, Vol. viii, p. 160. 



— — Emmons, Geol. Report, 1842, pag. 403, fig. 3. 



StropAomena. Id. lb. pag. 403, 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 Leptjena ; and the alternating character of the striae 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. altemata 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. 2 a. Mould of the interior of the convex valve of this species, showing t!'.e 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. 2 c. 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. 



