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.) 
Distribution, 15 species. Hurope, America, Africa. 
Fossil, 13 species. Hocene—. Britain, Germany. 
Famity TV.—ARcADz. 
Shell regular, equivalve, with strong epidermis; hinge with 
a long row of similar, comb-hke teeth; pallial line distinct ; 
muscular impressions subequal. Structure corrugated, with 
vertical tubuli in rays between the ribs or strive. (Carpenter.) 
Animal with the mantle open; foot large, bent, and deeply 
grooved; gills yery oblique, united posteriorly to a membranous 
septum. ; 
AroA, L. 
Etymology, arca, a chest. 
Tape, A. Now, Pl. X VIL. Bis. 12. 
Synonyms, Barbatia, Gray; Anomalocardia, Klein; Scapharca, 
Gray; Scaphula, Benson. 
Hxamples, A. granosa, Pl. XVIT., 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, iozenge- 
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; palpi0; gills long, narrow, less 
striated externally, continuous with 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 (Pl. 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 Arcas with close valves 
have the left valve a little larger than the right, and more 
ornate. 
The Bysso-arks secrete themselyes nder stones at low water, 
