105 



Isocardia vulgaris. PI. 36. Fig. 196. Shell, of the China type, show- 

 ing its concentrically ridged ivory-like texture, and delicately invo- 

 luted umbo. 



Family 6. CONCHACEA. 



Shell ; mostly orbicular or obliquely orbicular ; hinge composed of 

 three strong teeth, more or less, in each valve ; lateral teeth 

 sometimes wanting ; ligament external. 



In this family there is more variety and character in the shell than in 

 the animal. The animal is characterized throughout by a well-developed 

 foot, and by a pair of siphons which, according to the genus, are either 

 separated or united, with the margins sometimes plain, sometimes frilled. 

 The shell is mostly of solid porcellanous substance extremely rich in colour^ 

 decorated with strange fantastic patterns, and often beautifully sculptured 

 with spines, foliations, scales, and very elaborate lattice-work. The hinge 

 is composed of rather strongly developed teeth, and the ligament is ex- 

 ternal. The Conchacea are part marine and part fiuviatile ; the fluviatile 

 genera are limited in species, of sombre exterior, very little sculptured, and 

 there are rather strongly marked differences in the animal. 



Marine. 

 Tapes. CytherjEa. Venus. Artemis. 



LUCINOPSIS. ASTARTE. CyPRINA. 



Fluviatile. 

 Galath^a. Cyrena. Cyclas. 



Genus 1. TAPES, Megerle. 



Animal ; shaped as the shell, rather thick, having the mantle freely 

 open in front, its margins either plain or partially plain, or 

 if fringed, bordered by fine filaments ; siphons moderately long, 

 more or less separated, both branchial and anal orifices bor- 

 dered by cirrhi, those of the former ciliated ; foot lanceolate, 

 thick, with a byssal groove. (Forbes.) 



Shell ; equivalve, inequilateral, transversely oblong, sometimes ob- 

 liquely triangular, anterior side generally much the shorter ; 

 hinge composed of three contiguous teeth in each valve, of which 

 two are usually bifid ; no lateral teeth. 



VOL. II. p 



