96 



[ Senate 



nally, most ventricose near the central and umbonial region, 

 depressed and having a broad shallow flat or subangular mesial 

 sinus in front, terminating ( in old specimens) in a linguiform 

 extension : dorsal valve elevated and rounded along the middle, 

 declining laterally, having ( in old specimens) a strong rounded 

 mesial fold in front; beak incurved. Surface marked by ob- 

 scure traces of very fine radiating striae, which are crossed by 

 indistinct concentric lines of growth. 



This fine large species differs considerably in tlie form and depth of the 

 niesial sinus, as well as in the greater or less extension of the mesial pro- 

 longation of the anterior border of the ventral valve. In some of the spe- 

 cimens the sinus is very shallow and flattened within, while in other in- 

 stances it is more impressed and angular in the middle : other specimens 

 present intermediate grades of difference in this respect, the sinus being 

 nearly flat within, and marked by a narrow, nearly linear, deeper depression 



Merista princeps. 



