TRENTON LIMESTONE. 
109 
145. 8. LEPT2ENA ALTERNISTRIATA ( n. sp.). 
Pm XXXI. B. Figs. 1 a, b, c. 
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 
characters 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 
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 
striae. 
Fig. 1 b. Ventral valve of the same species, the striae being regular and uniform in size. 
Fig. 1 c. A smaller specimen, presenting a greater extension of the cardinal extremities, and an irregular 
alternation of the striae 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 
places, where it is associated with several other species. ( state Collection.) 
* The characters here given are pretty constant 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 strke and the prominent concentric lines. The distance of the prominent strise 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 that very variable species. 
