366 MOLLUSCA SUB-KINGDOM VI 
Arca, 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: Barbatia, Gray (Fig. 613); Scapharea, Noetia, Anadara (Fig. 614), and 
Argina, Gray ; Scaphula, Benson (fresh-water), ete. 
Isoarca, Mist. (Fig. 615). Shell smooth, inflated ; beaks full, incurved ; hinge 
line with rather amorphous dentition. Upper Jura and Lower Cretaceous. 
anda om 
TS Miuiswrauusn 

Fic. 615. 
7 BE Peal Arca (Isoarea) cordi- 
Fia. 613. i - . : = 
1a. 61 Fic. 614 formis, Ziet. Upper 
Arca (Barbatia) barbata, Linn. Mio- Arca (Anadara) diluvii, Lam. Jura; Nattheim, Wur- 
cene ; Grund, near Vienna. 1/;. Pliocene ; Sienna. temberg. 1/. 
Glycimeris, Da Costa (Pectunculus, Lam. Axinea, Poli), (Fig. 616). Rounded and 
almost symmetrical. Basal margin dentate ; area as in Arca, but shorter ; ligament 
A 


La 
a\ 


airy cw Tne 

MUI 
Il 
SS 

= 
Fie. 616. 
Glycimeris obovatus, Lam. Oligocene; Weinheim, near Alzey. 1/;. 
multivineular ; teeth oblique, in an arched series, interrupted during growth by the 
subsidence of the areal margin. Cretaceous to Recent ; maximum in Miocene. 
III. Scuizoponra. Steinmann (emend.) 
Superfamily 3. PTERIACEA. Dall. 
Shells of varied form, frequently alate, with a nacreous or sub-nacreous inner and pris- 
matic outer layer ; the epidermis seldom conspicuous ; area amplhidetic ; ligament variable, 
usually not parwincular ; gills filibranchiate or reticulate, usually reflected ; mantle lobes 
Sree, without siphons; pallial line simple; the anterior adductor smaller, or frequently 
dat ne 
