213 



in front for the reception of the extension of the oppo- 

 site valve; beak obtuse, strongly incurved. 



Surface marked by strong concentric lines of growth. 

 Interior of ventral valve marked by two strong diverging 

 dental lamellae which extend to near the middle and bound 

 a deep triangular muscular cavity. 



This species is most nearly related to Meristella (Merista) 

 tumida of European authors, but is less rotund, while that 

 species does not possess the peculiar flattening of the cardi- 

 nal half of the ventral valve, and its accompanying sub- 

 angular ridge. It differs from all the known species of the 

 Niagara and Lower Helderberg groups, but approaches in 

 some characters to the Meristella (Atrypa) crassirostra of the 

 Niagara group. {Pal. N. F., vol. H.) The M. nitida occurs 

 with this species, and is readily distinguished from it by its 

 emarginate front, and the absence of an elevated fold on 

 the dorsal valve. 



GENUS RHYNCHOSPIRA (Hall.) 



Rhynchospira evax (Hall). 



Shell ovate, often broadly ovate, usually longer, sometimes 

 much longer, than wide ; both valves gibbous in the mid- 

 dle and upper part, ventral valve a little deeper than the 

 opposite ; both valve sometimes marked by a shallow un- 

 defined sinus, causing an emargination in front ; ventral 

 beak much elevated above the other, and incurved, so as 

 to bring the plane of the foramen parallel to the axis of 

 the shell ; foramen distinctly rounded with a visible tri- 

 angular space below, which is occupied by two small del- 

 tidial plates. Dorsal valve regularly arcuate except near 

 the front, beak closely incurved beneath the deltidial 

 plates of the opposite valve. In some old specimens 

 there is a broad undefined mesial elevation on the lower 

 part of the valve, including about five or six plications 

 [Trans, iv.] 29 



