CONCHIFERA. 267 



Animal with the mantle closed ; byssal orifice 

 small; anal siphon very small, conical, plain, 

 branchial prominent, fringed inside ; palpi small, 

 triangular ; foot-muscles short and thick, close in 

 front of the posterior adductor. 



D. polymorpha is a native of the Aralo- Cas- 

 pian rivers; in 1824 it was observed by Mr. J. 

 Sowerby in the Surrey docks, to which it appears 

 to have been brought with foreign timber, in the Fi &- 18] - Breissena. 

 holds of vessels. It has since spread into the canals and docks of many 

 parts of the country, and has been noticed in the iron water-pipes of London, 

 incrusted with a ferruginous deposit. (Cunnington.} 



Fossil. 10 sp. Eocene . Brit. Germany. 



FAMILY IV. ARCADE. 



Shell regular, equivalve, with strong epidermis ; hinge with a long row 

 of similar, comb-like teeth ; pallial line distinct ; muscular impressions sub- 

 equal. 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. 



Etym. Area, a chest. Type, A. Note, PL XVII. fig. 12. 



Ex. A. granosa, PI. XVII. 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 grooved ; mantle 

 furnished with ocelli ; palpi ; 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 nume- 

 rous thin plates, occasionally becoming solid and calcarious ; 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 under stones at low-water, in crevices 

 of rocks, and the empty burrows of boring mollusks ; they are often much 

 worn and distorted. 



Listr. 130 sp. World-wide, most abundant in warm sea ; low water 



