LOWER HELDERBERG ROCKS. 249 



nerista siibqnadrata ( n. s.)- 



Plate XL. Fm. S a - d. 

 Shell subquadrate. Ventral valve the larger, gibbous in the middle and 

 umbonial region : beak prominent, incurved, apparently perforate. 

 Dorsal valve depressed convex : front slightly elevated, forming a 

 small undefined mesial prominence immediately on the margin : beak 

 well defined, incurved. 

 Surface smooth, or marked, with many indistinct concentric lines of 

 growth. 



The most marked characters of this species are its obUquely subquadrate form, 

 and the shght elevation of the front margin of the dorsal valve, without any trace 

 of a corresponding sinus in the opposite valve. 



Two well-marked specimens of this species only have been observed; but the 

 form and general characteristics are so well exhibited in both, that there is little 

 difficulty in identifying it among the more common forms of the genus. 

 Fig. 3 a. Dorsal view of this species. 



Fig. 3 b. Ventral view, showing the marks of the internal spires. 

 Fig. 3 c, d. Profile and front view of the same. 

 Geological position and locality. In the shaly limestone and in the lower penta- 

 merus limestone of the Lower Helderberg group : Schoharie and Carlisle. 



Merista arcnata ( n. s.). 



Plate XLI. Fia. \ a~t. 



Shell broad ovate, sometimes transversely oval. Ventral valve longitudi- 

 nally arcuate, gibbous in the central and umbonial region, having in 

 front a shallow- rounded depression which scarcely reaches the middle 

 of the valve : front margin ( in old specimens) elevated, and filling the 

 broad rounded sinus of the opposite valve. Dorsal valve often abruptly 

 elevated and very gibbous along the middle, and sloping laterally, 

 having no distinct mesial fold : beak incurved. 



Surface smooth, or marked by faint concentric lines and occasional 

 stronger wrinkles of growth, with faint radiating striae which are more 

 distinguishable upon the partially exfoliated shell. 



[ PAL-SONTOLOaT IIL] 32 



