62 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



Nymphs striated. Left posterior cardinal partly free and extending 

 across the hinge-plate ; left median triangular. Right cardinals 

 separate. Pallial sinus short and rounded (Eocene to Recent). 



Section Piiarina, J. -Br., 1913. Type, Cytherea citrina, Lam. 



Nymphs smooth ; left })osterior cardinal wholl}' free and oblique ; 

 outer cardinals of right valve united at top to form an arch ; pallial 

 sinus short, rounded, and ascending. 



Section Agriopoma, Dall, 1902. Type, Ci/therea texasiana, Dall. 



Shell dull white; nymphs smooth; left posterior cardinal long and 

 partly free ; outer cardinals of right valve forming a complete arch ; 

 pallial sinus sliarply angular. 



Sub-t/e^iera. 



Tinctora, n.n., J.-Br., 1914. Type, Cytherea vuhierata, Brod. 



Synonym: Callizofia, J.-Hv., 1913. 



Shell thick, sub-orbicular, glossy ; valve-margins crenulated ; left 

 posterior cardinal long and partly free from nymph ; median very 

 thick ; pallial sinus short and rounded. Pedal scar as in CaUista. 



Callocardia, A. Adams, 1864. Type, C. guttata^ A. Adams. 



Shell very thin. Hinge-plate narrow and excavated between the 

 teeth. Two cardinals in each valve, united to form complete curved 

 arciies. Left posterior cardinal long and free. Right posterior formed 

 of two narrow plates. Pallial line believed to be entire. 



Aphrodora, n.n., J.-Br., 1914. Type, Callocardia birt.si, Preston. 



Synonym : Leucothea, J.-Br., 1913. 



Shell thin, white. Hinge-plate short, curved, and narrowed 

 posteriorly. Teeth thin and weak ; left posterior sliort and confluent 

 with the nymph, right outer cardinals forming a complete arch. 

 Pallial sinus short and rounded. 



Atopodonta, Cossm., 1886. Type, Venus conformis, Desh. 



Shell small, but not thin. In tlie right valve the posterior cardinal 

 consists of two separate plates, one of which is united to the anterior 

 tooth, forming an arch over the median, which is bifid and A-shaped. 

 Pallial line entire. 



Genus LiocoNCHA, ^liirch. 



This genus was separated from Circe by Morch in 1853 and was 

 placed as a sub-genus of Meretrix by Fischer in 1887, but was adopted 

 as a genus by Dr. Dall (1902). I agree with Dr. Dall's view of its 

 taxonomic importance, for it is undoubtedly more nearly related to 

 CaUista than to Circe, and j'et it differs fiom the former in several 

 important particulars, as will be seen from the following definition. 



Type, Venus castrensis, Linn. 



Shell oval or sub-trigonal ; smooth or concentrically ridged. Lunule 

 superficial. Escutcheon not defined. Hinge-plate thick ; teeth like 

 those of CaUista, but left posterior long and partly free from the 

 nymph. Right posterior entire or feebly grooved. Dorsal margins 

 grooved as in CaUista. Pallial line entire. Pedal scar confluent with 

 that of the adductor. 



The genus includes two slightly different groups of shells ; the 

 one typified by castrensis is nearly smooth, the other typified by 



