424 MANUAL OF THE MOLLUSCA. 



to which it appears to have been brought with foreign timber, 

 in the holds of vessels. It has since spread into the canals, 

 docks, and rivers of many parts of England, France, and 

 Belgium, and has been noticed in the iron water-pipes of 

 London, incrusted with a ferruginous deposit. (Cunnington.) 



Distrihution, 15 species. Europe, Alnerica, Africa. 



Fossil, 13 species. Eocene — . Britain, Germany. 



Family IV. — ^Arcad^. 



Shell regular, equivalve, with strong epidermis ; hinge with 

 a long row of similar, comb-like teeth ; pallial line distinct ; 

 muscular impressions subequal. Structure corrugated, with 

 vertical tubuli in rays between the ribs or striae. (Carpenter.) 



Animal with the mantle open ; foot large, bent, and deeply 

 grooved ; gills very oblique, united posteriorly to a membranous 

 septum. 



Arca, L. 



Etymology, arca, a chest. 



Type, A. No^, PI. XYII., Fig. 12. 



;S?/?i0W2/??2S, Barbatia, Gray; Anomalocardia, Klein; Scapharca, 

 Gray ; Scaphula, Benson. 



Examples, A. granosa, PI. XYIL, Fig. 10. A. pexata. Fig. 11. 

 A. zebra. Fig. 13. 



Shell equivalve or nearly so, thick, sub-quadrate, ventricose, 

 strongly ribbed or cancellated ; margins smooth or dentated, 

 close or sinuated ventrally ; hinge straight, teeth very numerous, 

 transverse; umbones anterior, separated by a fiat, iozenge- 

 shaped ligamental area, with numerous cartilage-grooves; 

 pallial line simple ; posterior adductor impression double ; 

 pedal scars 2, the jDOsterior elongated. 



Animal with a long pointed foot, heeled, and deeply groved; 

 mantle furnished with ocelli ; palpi ; gills long, narrow, less 

 striated externally, continuous mth the lips ; hearts two, each 

 with an auricle. 



The name Bysso-arca was chosen unfortunately by Swainson, 

 for the typical species of the genus, in which the byssal orifice 

 is sometimes very large (PI. XVII., Fig. 13). The byssus is a 

 horny cone, composed of numerous thin plates, occasionally 

 becoming solid and calcareous ; it can be cast off" and re-formed 

 with great rapidity. (Forbes.) The Areas with close valves 

 have the left valve a little larger than the right, and more 

 ornate. 



The Bysso-arks secrete themselves nder stones at low water, 



