LOWER HELDERBERG ROCKS. 
249 
Merista subquadrata (n. s.). 
Plate XL. Fig. 3 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 obliquely subquadrate form, 
and the slight 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 arcuata (n. s.). 
Plate XLI. Fig. Is®/. 
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 strise which are more 
distinguishable upon the partially exfoliated shell. 
[ Palaeontology III.] 32 
