140 CRETACEOUS PELECYPODA 



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

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

 only find FetricolmHce in recent or in sub-recent deposits. 



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

 solid, with 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 ridge only ; in the left the middle tooth is thick and bifurcated ; the two others 

 very small and often almost obsolete; muscular impressions large, pallial sinus 

 deep, broad and obtuse. 



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

 the animal and shell of Choristodon^ of which CJi. divaricatum, Chem., is the type, 

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

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

 stodon is especially noticeable as compared with that of Petricola, Eossil species 

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

 rocks. 



3. Saxidomus, 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- 

 ally extending. The species of Sawidomus mostly are from the American seas, few 

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



4. Mupellaria, El. de Bell., 1802. Shell elongated, moderately tumid, 

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

 right valve with two cardinal teeth, and a third very small, but usually obsolete, 

 anterior; the middle one is prominent, curved as in Fetricola; the posterior is Ion-, 

 gitudinally 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. Bup, 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 Eleuriau de Bellevue to a species of P^^fwo/a and 

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

 have been unquestionable differences pointed out between El. 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 

 say that Eleuriau' 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-like forms of Venerupis. There 

 can be little doubt that the generic separation from Vejim'upis has a good ground ; 

 the hinge as compared with that of the compressed true Venerupis 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. 



