NIAGARA GROUP. 259 



geological position, as well as in the succeeding strata. (See the same in fossils of Lower Hel- 



derberg rocks.) estate Collection.) 



619. 25. LEPT.ENA STRIATA. 



PL. LIII. Fig. 7. 



Strophomena striata. Hall, Geol. Rep. 4th District, 1843, pag. 104, fig. 3. 

 Compare L. corrugata, page 59 of this volume. 



Shell semielliptical ; hinge-line equal, or a little longer than the width of the shell ; dorsal 

 valve slightly convex, ventral valve flat. Surface finely striated ; strioe crossed by concentric 

 strise, which are not always conspicuous. 



I have been unable to obtain specimens of this shell in a condition to examine the interior, 

 and I have some doubts regarding its specific distinction. The striai are apparently more round 

 than in L. subplana ; but in a few specimens I have detected some slight plications near the 

 hinge-line, which, with the round concentric strioe, give it the appearance of L. corrugata of 

 the Clinton group. 



All the specimens examined are extremely compressed, and closely adhering to the shaly 

 laminae. 



620. 26. LEPT^NA SUBPLANA. 



Pi.. LUI. Figs. 8-10. 



Strophomena subplana. Conrad, 1842, Jour. Acad. Nat. Science, Vol. viii, p. 258. 

 — — Hall, Geol. Rep. 4th Dist. N. York, 1843, pag. 104, fig. 1. 



Shell resupinate, semielliptical, length and width often nearly equal ; hinge-line extending 

 beyond the width of the shell. Surface marked by prominent sharp strice, which frequently 

 bifurcate before reaching the margin ; radiating striae crossed by strong concentric striae. Car- 

 dinal area extending to the extremities of the hinge-line, narrow, partially formed by both 

 valves. 



In a few well preserved specimens the sharp dichotomous striae are well preserved, having 

 the character described by Mr. Conrad ; while in other specimens, apparently of the same 

 species, the radiating striae are less prominent, and less conspicuously crossed by concentric 

 striae. The specimen fig. 10 a has precisely the characters of the one described by Mr. Conbad, 

 as I have ascertained by comparison ; but those of figs. 8 and 9 have more rounded striae, and 

 are somewhat proportionally shorter. I have not, however, been able to find decisive characters 

 for the separation of species. 



Fi^. 8 a. The interior of the convex valve of a young individual, where tlie width is greater 

 than the lengtli, the extremities of the hinge-line being extended beyond the width 

 of the shell below. 



Fig. 8 A. A specimen from which the shell is partially removed, showing, near the beak, the form 

 of the muscular impression, etc. 



