ADACNA. 459 



obovalis, Brod. and Sow. speciosa, Adams and Beeve. 



ovuloides, Beeve. tumorifera, Lam. 



planicostata, Sow. unedo, Linn. 



Sub-gen. ctenocardia, H. and A. Adams. 



Shell half-heart-shaped, the anterior side abruptly truncate ; 

 beaks carinated ; ribs spinose. 



Adamsii, Beeve. hjstrix, Brod. 



exasperata, Sow. imbricata, Soir. 



fornicata, Sow. virgo, Beeve. 



Sub-gen. lunulicardia, Gray. 



Shell closed, smooth or nearly smooth behind, cordate, keeled ; 

 lunule deeply impressed. 



auricula, Forsk. retusa, Linn. 



coucamerata, Wood. subretusa, Sow. 



Genus ADACNA, Eichwald. 



Siphons elongated, united nearly to their ends, the orifices 

 simple. Foot compressed. 



Shell thin, compressed, radiately grooved, gaping behind. 

 Hinge nearly edentulous. Pallial line sinuated. 



Si/n. Pholadomya, Agass. and Micld., not Sow. Hy- 

 panis, Pander. 



Ex. A. lseviuscula, Eichwald, pi. 112, fig. 4. Shell, A. 

 lseviuscula, fig. 4, a, 4, b. 



The species of this group are found in the Aral, Caspian, 

 Azof, and Black Seas, and in the embouchures of the Volga, 

 Dneister, Dneiper, and Don, burrowing in mud. 



