BATISSA. 9 



" Appears to be intermediate between C. cordata of Morris and 

 C. antiqua, both of which are PJocene species. From tlie llrst it 

 differs in being relatively higher ; its posterior end is also shorter, 

 and more distinctly subtruneate. From the latter it differs in 

 being less elevated, not so concave in front of the beaks, nor so 

 regularly arcuate on the posterior slope from the beaks to the 

 base. In front it presents the same symmetrical cordate outline 

 common to both these species." — M. & H. 



12. Corbiciila cytlieriforiiiis, Meek and Hayden.—" Shell 

 broad, trigonal, ovate, varying to subcircular, ratlier thick and strong ; ex- 

 tremities more or less rounded, base semi-ovate, usually more prominent 

 before than behind the middle ; dorsal outline sloping from the beaks, the 

 anterior slope being more abrupt than the other and slightly concave, 

 while the posterior is convex ; beaks rather elevated, moderately gibbous, 

 located in advance of the middle ; surface marked by fine lines of growth, 

 which sometimes show a very slight tendency to gather into small irregu- 

 lar concentric wrinkles." 



Hab. North America, near the mouth of the Judith River, Nebrnska. 

 Tertiary formation. (Cabinet of Smithsonian Institution.) 



Corbicula cytheriformis, M. & H. Proc. Ac. N. S. Phil. 18G1, 176. 



BATISSA, Gray.' 



Cyprina, Cyclas, Brug. 1792. — Cyrena, Lamarck, 1818. — Vetms, 

 Gray. — Batissa, Gray, 1852. 



Animal oval, transverse ; the lobes of the mantle simple or 

 fringed, united posteriorly ; two short syphonal tubes united at 

 their base ; foot compressed, oval, trigonal ; mouth transverse^ 

 tentacles elongate, oval, trigonal, free ; gills unequal, the inner 

 ones larger subquadrangular, outer ones subtrigonal. 



Shell oval, rounded or subtrigonal, thick, solid, hinge broad, 

 three unequal, divergent cardinal teeth in each valve, the posterior 

 tooth longest and narrowest ; lateral teeth unequal, double in the 

 right valve, narrow elongated, striated, anterior tooth shorter ; 

 ligament large, prominent, thick subcylindrical ; muscular impres- 

 sions large, lunular or circular; palleal impression simple. 



1 Ann. Mag. Nat. Hist., n. ser., IX, 1852, 34. 



