140 CRETACEOUS PELECYPODA 



strong geological eviclence for the separation into two genera, for while we meet 

 with typical Fetricolce already in the cretaceous (and probably Jurassic) period, we 

 only find Fetricolaria; in recent or in sub-recent deposits. 



2. Choristodon, Jonas, 1844. Shell ovate, tumid, truncate behind, rather 

 solid, -ndtli a rugously striated surface generally covered by a calcareous coating; 

 valves nearly quite closed ; hinge, strictly speaking, with three cardinal teeth in 

 each valve ; in the right the two anterior are distinct, the posterior is represented by 

 a low rido-e onlv ; in the left the middle tooth is thick and bifurcated ; the two others 

 verv small and often almost obsolete; muscvilar impressions large, pallial sinus 

 deep, broad and obtuse. 



Fischer (Jour, de Conch., vol. v, p. 323, &c.,) carefully examined and compared 

 the animal and shell of Choristodon, of which Ch. dicaricatum, Chem., is the type, 

 with those of Fetricola, and he considers the distinction between both to be a 

 o-eneric one. With regard to the animal the very small size of the palpi of Chori- 

 stodon is especially noticeable as compared with that of Fetricola. Fossil species 

 which in general form quite resemble the present genus already occur in cretaceous 



rocks. 



3. Saxidonms, Conrad, 1837. Shell ovate, ventricose, solid ; lunule and area 

 indistinct ; hinge with three to four cardinal teeth, the latter number occasionally 

 occurs in the right valve ; muscular impressions large, pallial sinus deep, horizont- 

 allv extending. The species of Snxidomus mostly are from the American seas, few 

 in number, and, strictly speaking, they are only a sub-division of Venerupis. 



4. Biipellaria, Fl. de Bell., 1802. Shell elongated, moderately tumid, 



surface rugously striated and ribbed, distinctly gaping posteriorly; hinge in the 



rio-ht valve with two cardinal teeth, and a third very small, but usually obsolete, 



anterior ; the middle one is prominent, curved as iu Fetricola ; the posterior is lon- 



i^itudinally laminar, low and bifurcate ; in the left valve are three distant and very 



unequal cardinal teeth ; the middle one is similarly projecting as the corresponding 



tooth in the other valve. Biq^. lamellifera, Conrad, may be considered as the 



type of the genus. Deshayes (Paris foss., 2nd edit., p. 402,) says that the name 



Bupellaria was applied by Fleuriau de Bellevue to a species of Fetricola and 



not a Venerupis. As the type of the last genus is well known, and as there 



have been unquestionable differences pointed out between Fl. de Bellevue' s 



Bupellaria and true Fetricola, it seems probable that that author had under view 



one of the shells for which the former name may be restricted. I am sorry to 



sav that Fleuriau' s opuscle is at present inaccessible to me, but even should my 



supposition not be entirely supported by the facts, I still believe that the former name 



can be secured with advantage for those Fetricola-liko. forms of Veiierupis. There 



can be little doubt that the generic separation from Veneriqns has a good ground ; 



the hin"-e as compared with that of the compressed true Veneriipis is really very 



different, more like that of Saxidomus and Fetricola, and equally distinct also is 



the shape of the shell, as I shall presently indicate more clearly. 



