l! 



114 PALJEONTOLOGY OF NEW<.YOEK, 



Fig. 6 a. View of ventral valve. 



Fig. 6 b. Cardinal view of the same shell. 



Position and locality. Associated with the last species, and others, in the lower shaly 

 limestone at Mineral Point ( Wisconsin). 



151. 14. LEPT^NA PLANOCONVEXA ( «. SJ3.). 



Pl. XXXI. B. Figs. 7 a, i, c, d. 



Shell resupinate, sub-planoconvex, often distinctly inequilateral and oblique ; ventral 

 valve moderately convex ; dorsal valve nearly flat or slightly concave ; cardinal line 

 extended considerably beyond the width of the shell, and often produced into small acute 

 ears ; surface marked by rather coarse radiating stria;, which bifurcate towards the margin 

 of the shell ; apex usually, and perhaps always, perforated ; foramen closed ; cardinal 

 area narrow. 



The most obvious characters of this shell are its resupinate form, the nearly flat dorsal 

 valve, and frequent inequality of the two sides, or the greater extension of the cardinal line 

 on one side. In these features alone it is dissimilar to all the other species of the genus in 

 the lower strata. In its nearly flat valve, it approaches to the Orthis ; while the extension 

 of the cardinal line, and closed foramen, are characters belonging to LEPT.a:NA, as well, 

 also, as its general habit, which is different from the true Orthis. 



In some of its varieties, and particularly in the ventral valve being flat near the beak, it 

 resembles the last species, but differs essentially from it in the coarser nearly equal strisp 

 and absence of concentric elevated lines, as well as other important characters. 



Fig. 7 a. View of the dorsal valve, with one of the cardinal extremities produced beyond the other ; tliis 



feature is not due to wearing, but is the natural form of the shell. 

 Fig. 7 b. View of a smaller specimen, more nearly equilateral. 

 Fig. 7 c. Profile view of the shell. 

 Fig. 7 d. Cardinal view of the first specimen. 



Position and locality. This species occurs associated with the two last, having a very 

 extensive geographical range. It is known in numerous localities in Ohio, Indiana, Ken- 

 tucky and Wisconsin. (Stale Collection.) 



