378 VENERID7E. 



in density, and in tlie almost entire absence of any trace 

 of animal matter. 



TAPES, Megerlb. 



This genus consists of a very natural assemblage of 

 Vemridfe, distinguished by marked characters of both shell 

 and animal, and by a general habit recognisable at a 

 glance. 



Shell solid, transversely oblong, or, more rarely, obliquely 

 subtriangular, equivalvc, inequilateral, closed, the posteal 

 extremity always longest ; surface smooth, or transversely 

 striated; inner margins smooth; muscular scars strongly 

 impressed, semicircular, the posteal always largest ; beaks 

 not prominent ; pallial sinus deep, oblong, rounded at the 

 extremity ; hinge composed of three scarcely diverging 

 primary teeth, two of which are usually bifid on each 

 valve, and a ridge bounding the groove for the ligament, 

 which is external. 



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 

 and not by strong scallops ; siphons moderately long, more 

 or less separated, sometimes for half their length, some- 

 times throughout ; both branchial and anal orifices bor- 

 dered by cirrhi, those of the former ciliated ; labial palps 

 rather long, lanceolate ; foot lanceolate, thick, with a 

 byssal groove, a structure which conspicuously distinguishes 

 this animal from Venus, and allies it, as well as the shape 

 of the shell, with Petricola. The species of Tapes inhabit 

 all climates, those of the tropics being most brightly co- 

 loured. Except the tertiary forms, most of the older 

 fossil species are doubtfully referred to this genus. They 

 inhabit the littoral, and, more rarely, the laminarian and 

 coralline zones. 



