102 [Assembly 



CRYPTONELLA(n.g.). 



Shells equilateral, inequivalve, elongate, oval or ovoid ; valves 

 unequally convex, without median fold or sinus, or with this 

 character moderately developed and principally towards the 

 base of the shell. Yentral valve with beak extended or 

 incurved, perforate ; foramen terminal, the lower side formed 

 by two small triangular deltidial pieces, or, in their absence, 

 by the umbo of the opposite valve. Shell-structure finely 

 punctate. Surface smooth, or with concentric stride. Valves 

 articulating by teeth and sockets, the dental lamellaB of the 

 ventral valve extending downwards into the cavity of the 

 shell. The muscular impressions in the dorsal valve are 

 strongly marked above, and extend in two narrow separated 

 impressions more than halfway to the front of the shell : the 

 ventral sinus shows elongated muscular and vascular impres- 

 sions. 



The species of this genus are more elongate than Merista and Meris- 

 TELLA, and those now known are less distinctly marked by mesial fold 

 and sinus; while the beak is more attenuate, often a little flattened, and 

 rarely so closely incurved, as in the genera cited. The punctate struc- 

 ture of the shell is a distinguishing character. 



CENTRONELLA IMPRESS A ( n. s.). 



Shell subtriangulai-ly ovoid, elongate ; width and length about 

 as seven to nine. Yentral valve convex, much elevated or 

 subangular in the middle, with sides abruptly sloping to the 

 margins, sinuate in front ; beak extended and nearly straight. 

 Dorsal valve slightly concave in the upper and middle por- 

 tions, with usually an impressed line down the centre, abruptly 

 inflected at the sides and less abruptly in the front, which is 

 produced in a short process filling the sinus of the opposite 

 valve. Surface finely striate concentrically, with faint remains 

 of extremely fine radiating strias, which are visible only under 

 a lens : entire surface finely punctate. 



The interior of the ventral valve shows two strong teeth much 

 below the beak, with an elongate triangular foramen reach- 



