No. 109.] 



95 



Merista arcuata ( n. s.). 



Pal. N.Y. Yol.iii, pi. 41, f. 1. 



Shell broad-ovate, sometimes transversely oval : ventral valve 

 longitudinally arcuate, gibbous in the central and unibonial 

 region, having in front a shallow rounded depression scarcely 

 reaching the middle of the valve ; front margin ( in old speci- 

 mens) elevated, and fitting into the broad rounded sinus of the 

 opposite valve : dorsal valve often abruptly elevated 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. 

 Some varieties of this species bear considerable resemblance to the last : 

 they are, however, always ventrieose and proportionally broader. The 

 ventral valve is also more arcuate longitudinally, more distinctly sinuate, 

 and elevated at the front. The dorsal valve is likewise more compressed 

 in this species, especially near the lateral margins. 



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

 berg group, Albany and Schoharie counties. 



MiKISTX AKCUATJL. 



Merista princeps (n.s.). 



Pal. N.Y. Yol.iii, pi. 44, f.1-5. 



Shell ovate ; sides sloping towards the beaks at an angle of about 

 30° : ventral valve more or less profoundly arcuate longitudi- 



