CAIiCABEOUS. 91 



BTSSOxiECA. Sw. 1835. (Byssus and Area.) Fam. Ara.cea. Lam. A 

 genus of bivalve shells, composed of the Area Noce, and several 

 other species, separated from the genus Area on account of their 

 shells being attached by means of a byssus passing through an 

 hiatus in the ventral margins. B. Noce, fig. 132. The species 

 occur in Southern Europe, East and "West Indies, China ; also, 

 on the coasts of Grreat Britain. PL vii. fig. 132. 



BTSSOMTA. Payr. 1826. (B^/ssus and Mi/a.) Be Blainville 

 states that, although the shell of this proposed genus resembles 

 Saxicava, the animal is sufB.ciently difierent to justify the sepa- 

 ration. 



BYSSUS. (BvarcToe, byssus, ancient name for linen.) The ten- 

 dinous fibres by which some Bivalves are as it vrere anchored or 

 moored to submarine substances. A fine example of this is to 

 be seen in the Pinnae, which bear some resemblance to large 

 Mussel Shells, and have an hiatus in the margin of the valves, 

 through which a bunch of silken fibres passes. In the British 

 Museum there is preserved a pair of gloves, which have been 

 woven of these fibres. The Byssus is peculiar to some bivalve 

 shells, such as Mussels, Hammer Oysters, Area Nose, &c. 



C^CUM. Fleming. A genus of minute sheEs resembling Den- 

 talia, as if truncated, &c. the opening filled by a kind of septum. 

 The position of this genus in the system is not yet accurately 

 ascertained. Caecum trachea, British Mollusea, PI. Ixix. fig. 4 ; 

 our figure, PL xxvii. fig. 565. 



CALCAE. Montf. 1817. (a spin-.) A genus composed of Tbo- 

 CHUS STELLAEis, Lam. and other depressed species of Trochus 

 which are characterized by a stellated keel round the angle of 

 the last whorl ; but not includiag T. Imperialis, which is the 

 genus Imperator, Montf. The difierence consists in the latter 

 being umbilicated, and the former not. T. stellaris, fig. 358. 



CALCAEEOUS. (calx, lime.) A term applied to a shell, or to 

 its operculum, which is composed principally of lime or shelly 

 matter, as is usually the case, in distinction from one which is 

 of a horny, membranaceous texture. The greater number of 

 shells are calcareous, but it forms an important point of distinc- 



