474 



MOLLUSCA 



PHYLUM YI 



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

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

 more com})lete. 



Crassatellites Kruger {Crassatella Lam. 1819, non Lam. 1799) 

 (Figs. 759, 760). Cretaceous to Eeceiit ; represented by about seventy 

 fossil and forty living species. 



Trioclonta Koenen. Oligocene. Scavibula Conrad ; Remondia 

 Gabb {Stearnsia White) ; Antlwnyia Gabli ; Grassatellina ISIeek. 

 Cretaceous. 



(?) Ptychomya Ag. Like Crassatellites, but with radial sculpture 

 "" ''^^' and three cardinals in each valve. Cretaceous. 



hronni ' Merlm Grassinella Guppy, non Bayle {Gouldia auct., non Adams ; 



Oligocene; Wein- Pseiideripliyla Fisch.). Small, sub-triangular, very compressed, con- 



nfjiiiij iiGtir Aiz(*y. * ti •tit m • i t\ 



i/j. centrically ribl:)ed. Tertiary and Kecent. 



Superfamily 3. CYRENACEA Tryon. 



Cyjiricardians which have become specialised^ for fresh or brackish water conditions, 

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

 parous. 



Family 6. Oyrenidae Gray. 



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

 valves equal, free, closed, usxially with plain margins; area obscure or none; ligament 

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

 from the pedal ; pallial line simple or ivith a small simis ; 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 s}>ecies are generally without it. In 

 some fossils the laterals of the right valve are not double. 



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

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

 maximum in the Cre- 

 taceous and onwards. 



Subgenera : Corbieula 

 Megerle (Figs. 761, 762). 

 Smaller than Cyrena, and 

 the laterals sharply cross- 

 striated. Ecjcta Adams. 

 Compressed, elongated, 



thin ; almost rostrate. 

 Recent ; marine. 



Like 



Fic. 761. 



right 



Corhicula fluminalis 

 (MiilL). Pleistocene ; 

 Teutschentlial, near 

 Halle, Saxony. 



Corhicula semistriata Desh. Oligocene 

 (Cyrena marls) ; Flonheini, near Alzey. Vi- 



Batissa Gray. 

 Cyrena, but the 



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

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

 region. 



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

 Oligotiene to Recent. The relations of the Recent Egeria Roissy, and Proflscheria Dall 

 {Galatea Lmg., non Fabr. ; Fischeria Bernai'di, non Desv.) are not positively fixed. 



