TRENTON LMESTONE. 109 



145. 8. LEPT^NA ALTERNISTRIATA (n. 5p.)- 



Pi.. XXXI. B. Figs. 1 a, b, e. 



Shell semioval, wider than long ; cardinal line frequently extended beyond the width 

 of the shell ; dorsal valve moderately convex in the middle, and gradually curving up- 

 wards ; surface marked by radiating striae of unequal size, a large and small one often 

 alternating ; ventral valve with nearly equal radiating striae ; entire surface marked by 

 fine concentric elevated lines ; cardinal area narrow, almost linear ; callosity of the ventral 

 valve nearly filling the foramen of the other ; beak perforated by a minute nearly micro- 

 scopic circular opening. 



It will be observed that this shell possesses several characters in common with L. alter- 

 nata ; but it appears to me somewhat different, and I am not able to find a gradation in the 

 char.icters which would lead me to unite it with that species. It is usually more extended 

 on the cardinal line than that species, and has in consequence a different form, as in 1 c. 

 The shell is always lighter and apparently thinner, less abruptly curved and thickened on 

 the margin, though often reaching the size of the largest varieties of L. alternata. The 

 striae on the ventral valve are always of uniform size, while, on the dorsal valve, they 

 alternate in size, often very regularly. This form of shell is usually sufficiently distinct ^^fen 



to enable me to separate it readily from among the various forms of the L. alternata, and, 

 on this account, I have proposed a distinct name. 



Fig. 1 a. The dorsal valve of a large specimen, showing a very regular alternation in the size of the 



strije. , 



Fig. 1 b. Ventral valve of the same species, the strise being regular and uniform in size. 



Fig. \ e. A smaller specimen, presenting a greater extension of the cardinal extremities, and an irregular 

 alternation of the stri® upon the surface.* 



Position and locality. I have not been able to discover this species well characterized 

 among the shells of the Trenton limestone in New-York, though it is not rare in the Blue 

 limestone of the West, at Cincinnati (0.), Maysville (Ky.), Madison (la.), and other 

 place.s, where it is associated with several other species. (State Collection.) 



* Tlie characters here given are pretty conatant in a large number of specimens ; but I have found, since the 

 engravings were finished, a variation in some individuals. A single specimen presents characters like fig. 1 n, pi. 30, 

 in the distance of the striae and the prominent concentric lines. The distance of the prominent striae is also much 

 greater in this one, and yet there is still but a single intermediate stria. The species, if distinct, suffers many changes 

 and variations analogous to the L. alternata, and it may yet prove a variety of tliat very variable species. 



