141 



The ventral valve is much thickened on each side towards 

 the beak, and the rostral cavity margined by flattened den- 

 tal lamella), which extend downwards to the commence- 

 ment of the muscular impression, and terminate at the 

 edge of the shell in blunt tooth-like processes. The mus- 

 cular impression forms a somewhat broadly triangular de- 

 pression in the valve just below the rostral cavity. In the 

 cast of this valve we have the reverse of these features. 



In the dorsal valve there is a strong hinge plate or pro- 

 cess, the prominent part of which is broadly triangular, 

 somewhat depressed or spoon shaped in the centre, and 

 supported below by a median septum which reaches from 

 one-third to one-half the length of the valve, and on each 

 side, marked by deep dental fossets, while the anterior 

 angles are produced into the crura which support the in- 

 ternal spires. 



Spires arranged as in Athyris and Merista, being a 

 double cone with the apices directed outwards. From the 

 lower lateral margins of the cardinal process or hinge 

 plate, there is a callosity extending beneath and anterior 

 to the dental fossets, and joining with the thickened margin 

 of the valve, as in the other allied genera. 



Iu the cast of the dorsal valve we have the mark of the 

 median septum, with an elongate, lanceolate muscular im- 

 pression, reaching nearly to the middle of the valve. The 

 imprint of the triangular process, and the cavities made 

 by the crura are often preserved. 



The species of this genus may be readily distinguished 

 from Meuista by the absence of the shoe-lifter process, 

 which, in numerous specimens compared, constitutes the 

 principal difference between the two genera. 1 



The following illustrations will serve to show more clearly 

 the characteristics of the genus : 



1 On plates 39 and 41 of Pal. N. Y., vol. ni, may be found some illustra- 

 tions of the casts of species of this genus. 



[Trans, iv. ] 20 



