366 



MOLLUSCA 



SUB-KINGDOM 



Area, Lam. (type A. Noae, Lin.) Shell trapezoid,.equivalve, with a wide amphi- 

 detic area, distant conspicuous beaks, and radial sculpture ; a wide byssal gape ; a 

 long, straight, transversely dentate hinge line, with many small similar teeth. 

 Tertiary and Recent. Used in the wider sense, to include all the groups of Arcidae, 

 there are some 200 living and 300 to 400 fossil species. 



Sub-genera: Harbatia, Gray (Fig. 613); Scapharca, Noetia, Anadara (Fig. 614), and 

 Argina, Gray ; Scaphula, Benson (fresh-water), etc. 



Isoarca, Miinst, (Fig. 615). Shell smooth, inflated; beaks full, incurved; hinge 

 line with rather amorphous dentition. Upper Jura and Lower Cretaceous. 



Arm (Jlnrljutia) barbata. Linn, 

 cene ; Grund, near Vienna, 1/1- 



Mio- 



FIG. 614. 



Area (Anadara) diluvii, Lan 

 Pliocene ; Sienna. 



FIG. 615. 



Area (Isoarca)} cordi- 

 formis, Ziet. ' Upper 

 Jura ; Nattheim, Wur- 

 teinberg. 1/1- 



Glycimeris, Da Costa (Pectunculus, Lam. Axinea, Poli), (Fig. 616). Rounded and 

 almost symmetrical. Basal margin dentate ; area as in Area, but shorter ; ligament 



FIG. 616. 

 (flycimeris obovatus, Lam. Oligocene ; Weinheim, near Alzey. 1/1- 



multivincular ; teeth oblique, in an arched series, interrupted during growth by the 

 subsidence of the areal margin. Cretaceous to Recent ; maximum in Miocene. 



III. SCHIZODONTA. Steinmann (emend.) 

 Superfamily 3. PTERIACEA. Ball. 



Shells of varied form, frequently alate, with a nacreous or sub-nacreous inner and pris- 

 matic outer layer ; the epidermis seldom conspicuous ; area amphidetic ; ligament variable, 

 usually not parivincular ; gills filibranchiate or reticulate, usually reflected ; mantle lobes 

 free, without siphons ; pallial line simple ; the anterior adductor smaller, or frequently 



