No. 109.] 97 



rlong its middle. Young individuals are almost entirely destitute of a sinus, 

 though the front of the dorsal valve in such cases is usually slightly ele- 

 vated. 



Geological posificn and locality. Upper Pentamerus limestone of the 

 Lower Helderberg group, Carlisle and Schoharie. 



Merista MEEKI. 

 Pal. N.Y. Vol. iii, pi. 44, f. 6. 



Shell cordate-ovate, very gibbous, sloping from below the middle 

 towards the beaks at an angle of about 80° : ventral valve pro- 

 foundly arcuate from the beak to the anterior margin, where it 

 terminates in a prominent abruptly tapering mesial prolonga- 

 tion, having a broad angular sinus from near the beak quite 

 to the termination of the anterior prolongation ; beak flat- 

 tened on the outside, subangular on its lateral slopes, closely 

 incurved upon that of the opposite valve : dorsal valve elevated 

 along the middle, sloping laterally with an abrupt curve, very 

 gibbous in the umbonial region; beak incurved. Surface smooth, 

 or marked by faint concentric lines of growth. 

 The shorter and more globose form of this species, as well as the deep 



angular sinus and flattened umbo of its ventral valve, are sufficient to 



distinguish it from all the preceding species. 



Geological position arid locality. Limestone of the age of the Lower 

 Helderberg, Tennessee. 



Meganteris mutabilis. 



Pal. N.Y. Vol. iii, pi. 45, f: 2. 



Shell ovate varying to elliptic and obovate, not sinuate on eilher 



valve ; old specimens sometimes very gibbous, but generally 



compressed towards the anterior border in young individuals ; 



valves nearly equally convex : ventral valve most elevated 



I Senate No. 109.] 7 



