ARCA. 233 



to be the case, we cannot admit well known and character- 

 istic drift species into our list of living forms, and in this 

 category we must place Leda oUonga {Nucula ohlonga. 

 Brown, 111. Conch. G. B. p. 84, pi. 33, f. 1) and Leda 

 truncata {Nucula truncata, loc. cit., p. 84, pi. 33, f. 1), 

 both of which are, however, still found living in Arctic 

 regions. 



ARCA. LiNN^us. 



Shell more frequently equivalve, more or less inequila- 

 teral, closed or gaping in front, more or less quadrate, 

 usually solid, almost always radiatingly ribbed or striated, 

 covered with an epidermis, which is in most instances 

 loose and rough ; margin smooth or crenate ; beaks remote, 

 separated by a tetragonal grooved ligamental area. Hinge 

 straight or nearly so, consisting of small interlocking teeth, 

 parallel in the centre, oblique at the sides : ligament ex- 

 ternal. Pallial impression entire, muscular scars very 

 marked, and sometimes ledge-shaped. 



Animal oblong, mantle freely open, simple or fringed ; no 

 siphons. Foot large, oblong, bent, grooved throughout its 

 length, so as to form a disk with plain or slightly crimped 

 margins ; a byssal gland at its base. Byssus compact. 

 Mouth surrounded by labia formed out of the extremities 

 of the branchiae ; no true palps. 



The Arks, so called from their boat-like shapes, are 

 mostly dwellers in crevices of rocks and cavities of shells, 

 moored by their powerful byssus, but can equally live free. 

 They are found in all depths of water, though the majority 

 of species are littoral. The genus has most representatives 

 in the Tropics. M. Nyst, who has recently catalogued both 

 the existing and extinct forms of Area, enumerates nearly 



VOL. II. H H 



