NIAGARA GROUP. 



259 



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

 derberg rocks.) (^State Collection.) 



619. 25. LEPT^NA STRIATA. 



t 



PL. LIII. Fig. 7. 



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

 Compare £,. 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 ; strise crossed by concentric 

 strice, 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 strise 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 stride, 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.ENA SUBPLANA. 



PL. 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 striae, which frequently 

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

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

 valves. 



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

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

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

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

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

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

 for the separation of species. 



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

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

 of the shell below. 



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

 of the muscular impression, etc. 



