108 [Senate 



Leptoccelia imbricata. 



Pal. N.Y. Vol. iii, pi. 38, f.8 to 12. 



Shell longitudinally scraielliptical or suborbicular : ventral valve 

 convex, most prominent along the middle, and sloping laterally; 

 beak small, incurved at the apex and perforated by a very small 

 round aperture, one side of which is formed by the deltidium : 

 dorsal valve tiattened or depressed-convex; beak scarcely ele- 

 vated above the hinge ; hinge sloping from the beaks at an angle 

 of abr/ut 150°, rounded at the extremities, nearly equal to the 

 greatest width of the shell ; area narrow, shorter than the hinge. 

 Surface marked by ten to twelve plications on each valve, of 

 which the two on the middle of the ventral valve are larger and 

 more elevated than the others, and separated by a wider and 

 deeper depression than between those on each side. The central 

 plication on the dorsal valve is larger than the others near the 

 front, but usually dies out before reaching the beak. Shell 

 marked by strong imbricating concentric lamellse of growth. 



This species resembles Leptocalia (AtrTpa) disparilis of the Niagara 

 group, but is generally larger, has more plications and a straighter hinge ; 

 its dorsal valve also differs, in being slightly convex instead of concave. It 

 also bears a general resemblance to Terehratvla /epida (Goldftjss) and 

 T. sublepida (DE Verneuil), but has a wider hinge than either of these, 

 and differs likewise from them in other obvious and essential characters. 



Geological position and locality. Shaly limestone of the Lower Helder- 

 berg group, Albany county. 



Leptocgelia propria. 



« 



Pal. N.Y. Vol.iii, pi. lOG, f. 1. 



Shell somewhat semicircular, varying to suborbicular or trans- 

 versely oval, generally broader than long : ventral valve con- 

 vex, most prominent along the middle, declining laterally; 

 beak incurved, with a small round perforation in the extremity, 

 which is completed on the lower side by the two dcltidial pieces : 



