202 CRETACEOUS PELECYPODA 



anterior of tlie three cardinal teeth, it is nearly horizontal ; pallial sinus short, but 



distinct. 



4. Batissa, Gray, 1852. Shell broadly cordate, moderately compressed; 

 cardinal teeth bifid, lateral teeth elongated, on the outer side only, or sometimes 

 on both sides ; cross grooved ; pallial impression usually truncate posteriorly. The 

 shells of Batissa generally are somewhat compressed ; they may be regarded as an 

 intermediate form between Cijrena and Corhicula, agreeing with the former in the 

 usual larger size and the habitat in brackish waters, with the latter in the elongated 

 form of the lateral teeth, although the anterior lateral tooth is perceptibly shorter 

 than the posterior one, while in Covblcula both are very nearly equal in length 

 and much compressed. 



5. Corbicula, Meg., 1811. Shell cordate, moderately inflated, with three more 

 or less distinctly bifid, cardinal teeth, and one much elongated, compressed, and 

 transversally finely striated lateral tooth on either side; pallial line posteriorly 

 truncated, or with a short sinus. 



The anterior cardinal tooth of the right and the posterior cardinal of the left 

 valve are in some species very small, and especially the latter sometimes almost 

 obsolete. This makes it difficult in generically separating the Wealden forms, 

 although they almost invariably seem to possess only two cardinal teeth in each 

 valve. 



6. Isodoma, Desh., ISGO, (Paris foss., 2nd edit., i, p. 481). Shell thin, fragile, 

 resembling a dementia, elongately ovate, moderately tumid ; hinge of right valve 

 with two bifid diverging cardinal teeth, and one distinct remote lateral tooth on 

 either side ; pallial sinus not known. 



This is based upon a fossil species, I. cyrenoides, from the Paris basin ; in 

 general character the shell recalls dementia and some other allied forms with a 

 thin shell, which have been provisionally referred to the dosiniin^, but the denti- 

 tion of the hinge of Isodoma has its only analogue among the Cyshnid^. 



7. Fisidium, Pfeiff., 1821, Shell ovate, moderately inflated, beaks tumid ; 

 hinge with two diverging more or less distinctly bifid cardinal teeth, and one lateral 

 tooth on either side in each valve ; pallial impression entu'c, truncate posteriorly ; 

 ligament placed always on the shorter side of the valves. Siphons of the animal 

 united to the end, short. There are usually two cardinal teeth in each valve, the 

 posterior in the right and the anterior in the left are mostly bifid, the two others 

 single ; in very aged specimens the cardinals occasionally become nearly obsolete. 



8. SplKcrimn, Scop., 1777, (Cyclas, Brug.). Shell ovate, generally somewhat 

 elongated, moderately inflated, beaks tumid, but slightly prominent ; hinge usually 

 with two, short, closely approached cardinal, and two prominent lateral, teeth, one 

 on each side ; pallial impression entire ; ligament placed always on the longer side 

 of the valves. Animal with the siphons elongated and separate. 



Setting aside the distinction existing between the animals of the two last 

 genera, there appears to be a remarkable difference in the position of the cardinal 

 teeth. In Fisidittm the cardinal teeth are elongated, diverging, and when the 



