474 



MOLLUSCA 



PHYLUM TI 



resilium has become large and deeply immersed. The parallelism between this group and 

 the Mactridae, in the gradual inimersion of the ligament, could hardly be 

 niore complete. 



Grassatellites Kruger {Grassatella Lam. 1819, non Lam. 1799) 

 (Figs. 759, 760). Cretaceous to Kecent ; represeiited by about seventy 

 fossil and forty living species. 



Triodonta Koenen. Oligocene. Scamhula Conrad ; Bemondia 

 Gabb {Stearnsia White) ; Änthonyia Gabb ; Grassatellina Meek. 

 Cretaceous. 



(?) Ptychomya Ag. Like Grassatellites, but witli radial sculpture 

 and three cardinals in each valve. Cretaceous. 



Grassinella Guppy, non Bayle {Gouldia auct., non Adams ; 



Fk 



Grassatellites 

 hronni Merian. 



Oligocene; Wein- Pseuderiphyla Fisch.). Small, sub -triangulär, very compressed, con- 

 Vi. centrically ribbed. Tertiary and Kecent. 



Superfamily 3. CYRENACEA Tryon. 



Gypricardians which have become specialised for fresh or brackish water conditions, 

 and, as usual in such cases, have developed great variability of character ; usually vivi- 

 parous. 



Family 6. Oyrenidae Gray. 



Shell porcellanous, with a conspicuous epidermis, usually with concentric sculpture ; 

 valves equal, free, closed, usually with piain margins ; area obscure or none ; ligament 

 and resilium external, parivincular, opisthodetic ; adductor scars sub -equal, separate 

 from the pedal ; pallial line simple or ivith a small sinus ; hinge with anterior and 

 posterior laterals usually double in the right, single in the left valve, distinctly separated 

 from the cardinals ; cardinal teeth bifid. at the summit, three in each valve when none 

 are obsolete. Mantle open ventrally, the siphons distinctly developed, short, more or less 

 united. Lias to Recent. 



Many of these forms merge with one another as'we recede in time. The Recent American 

 forms and many fossils show a pallial sinus ; oriental species are generali y without it. In 

 some fossils the laterals oF the right valve are not double, 



Gyrena Lam. Rounded, sub-equilateral, plump, concentrically sculptured, with 

 smooth margins ; cardinals three, the laterals smooth. Lias to Recent (300 species) ; 

 maximum in the Cre- 

 taceous and onwards. 



Subgenera : Corbicula 

 Megerle (Figs. 761, 762). 

 Smaller than Gyrena, and 

 the laterals sharply cross- 

 striated. Egeta Adams. 

 Compressed, elongated, 



thin ; almost rostrate. 

 Recent : marine. 



Fig. 761 



CorMcula fluminalis 

 (Müll.). Pleistocene ; 

 Like Teutschenthal, near 

 . Halle, Saxony. 



right 



Fir.. 762. 



Corbicula semistriata Desli. Oligocene 

 (Gyrena marls) ; Flonheim, near Alzey. i/^. 



Batissa Gray. 

 Gyrena, but the 



anterior and left posterior cardinals feeble or obsolete ; anterior laterals very short, 

 posterior ones elongated. Upper Cretaceous of Oregon, and living in Indo-Pacific 

 region. < 



^ Veloritina, Leptesthes Meek. Laramie Group. Villorita Gray {Velorita Gray, 1 847). 

 Oligocene to Recent. The relations of the Recent Egeria Roissy, and Profischeria Dali 

 {Galatea Brug., non Fabr. ; Fischeria Bernardi, non Desv.) are not positively fixed. 



