98 [ Senate 



near the middle and towards the umbo ; beak pointed, sub- 

 angular along the lateral slopes, arched or closely incurved ; 

 foramen narrow, and extending nearly or quite to the apex of 

 the beak : dorsal valve slightly less elevated and a little shorter 

 than the opposite ; beak not projecting, incurved. Surface 

 marked by twelve to twenty -eight coarse obscure radiating striae, 

 crossed by fine indistinct lines of growth, and sometimes near 

 the border by a few strong concentric undulations. The radiating 

 strise are usually obsolete on the upper half of the shell. 



This species varies considerably in form, as well as in otlier characters. 

 Young individuals are generally more compressed near the front, and the 

 beak is more nearly straight ; while older specimens are often quite gib- 

 bous, and sometimes marljed by very strong concentric undulations : in 

 the latter case, the beak of the ventral valve is generally closely incurved. 



Geological position a7id locality. Higher part of the Shaly limestone, 

 and more compact beds just beneath the Upper Pentamerus limestone of 

 the Lower Helderberg group, Albany and Columbia counties. 



Meganteris elliptic a. 



Pal. N.Y. Vol. iii, pi. 45, f. 4. 



Shell elliptical, rather gibbous ; valves nearly equally convex ; 

 front rather sharply rounded ; no trace of a sinus on either 

 valve : dorsal valve most elevated near the middle, rounding 

 laterally, and having a semielliptical outline from the front to 

 the beak, which is incurved : ventral valve curving from the 

 middle towards the lateral margins, and forming longitudinally 

 a semielliptic curve; beak rather gibbous, closely incurved 

 and extended over that of the other valve. Surface marked by 

 fine indistinct radiating strise, which are crossed by obscure 

 remains of concentric lines and faint undulations of growth. 



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

 berg group, Albany county. 



