TRENTON LIMESTONE. 109 



145. S. LEPTiENA ALTERNISTRIATA (H.iyj.). 



Pl. XXXI. B. Fii?s. 1 a, i, r. 



Shell seiiiioval, wider than long ; cardinal line frei|iiently exieiuied beyond the width 

 of the shell ; dorsal valve moderately convex in tlie niiddh^, and gradually curving up- 

 wards ; siuface marked by radiating stria; of unequal size, a large and small one often 

 alternating ; ventral valve with nearly equal radiating stri.-e ; entire surface marked by 

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

 valve nearly filling tlie fcnanien of tlie other ; beak perforated by a minute nearly micro- 

 scopic circular openiiitf. 



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

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

 characters wliich 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 

 (he maru^in, though often reaching the size of the largest varieties of /.. alternata. The 

 strife on the ventral valve are always of uniform size, while, on the dorsal valve, (hey 

 alternate in size, often very regularly. This form of shell is usually suthciently 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 speciinen, showing a very regular alternation in the size of the 



striffi. 

 Fig. 1 h. Ventral valve of the same species, the stria; being regular ami uniform in size. 

 Fig. 1 c. A smaller specimen, presenting a gpreater extension of the cardinal extremities, and an irregular 



alternation of the striffi upon the surflicc.* 



Position and localify. I have not been able to discover (his 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 (O.), Maysville (Ky.) , Madison (la.), and other 

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



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

 engravings were finished, a variation in some individuals. A single sjiecimen presents characters like fig. t 7i, pi. 30, 

 in the distance of the strife and the prominent concentric lines. The distance of the prominent stritE 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. 



V- 



