424 MANUAL OF THE MOLLtJSCA, 



to whicli 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.) 



Distribution^ 15 species. Europe, America, Africa. 



Fossil^ 13 species. Eocene^ — . Britain, Grermany. 



Family IT. — Aecad^. 



Sliell 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.) 



^mwaZ with the mantle open; foot large, bent, and deeply 

 grooved ; gills very oblique, united posteriorly to a membranous 

 septum. 



Aeca, L. 



Etymology, area, a chest. 



Tijpe, A. Nose, PL XVII., Pig. 12. 



8yno7iyms,^arha.tisii Grray; Anomalocardia, Klein; Scapharca, 

 Gray ; Scaphula, Benson. 



Examples, A. granosa, PL XVIL, 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 flat, lozenge- 

 shaped ligamental area, with numerous cartilage-grooves; 

 pallial line simple ; posterior adductor impression double ; 

 pedal scars 2, the posterior elongated. 



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

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

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

 with an auricle. 



The name JBysso-arca was chosen unfortunately by Swainson, 

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

 is sometimes very large (PL XYIL, 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 mor( 

 ornate. 



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



