CONCHIFERA. ' 267 



Jnimal 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. 



B. polymorpJia is a native of the Aralo-Cas- 

 pian rivers; in 1824 it was observed by Mr. J. ^ " ]1V^^ 

 Sowerby in the Surrey docks, to which it appears 

 to have been brought with foreign timber, in the ^^S- ^^^- Dreissena. 

 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, (fiunnington) 



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 

 striffi. {Carpenter.) 



Animal ^iih. i]iQ mantle open; foot large, bent," and deeply grooved; 

 gills very oblique, imited posteriorly to a membranous septum. 



Arca, L. 



Mym. Arca, a chest. Type, A. Noa, PI. XVII. fig. 12. 



Ilx. 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 am'icle. 



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 nume- 

 rous thin jjlates, occasionally becoming solid and calcarious; it can be cast 

 off and re-formed with great rapidity, {Forbes.) The Areas \vith close valves 

 have the left valve a little larger than the right, and mere 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. 



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



