Atrina.] PELECYPODA. 893 



Shell equivalve, triangular, wedge - shaped, exceedingly inequi- 

 lateral, with the beaks pointed, anterior, terminal ; the apical portion 

 entire, not fissured, the internal nacreous layer entire. Hinge straight, 

 long, toothless. Ligament linear, internal. Pallial line entire. 



Distribution. Warm and temperate seas, from low-water mark to 

 about 100 fathoms. 



Fossil the genus appears first in the Carboniferous. 



Remarks. The animals live buried in mud or sand, with the sharp 

 edges of the valves gaping and protruding from the surface. Pearls 

 of an amber colour are sometimes found in the shells of Pinna and 

 Atrina. Some species are used as food, and others are valued for 

 their long and silky byssus, which, mixed with silk, has been woven 

 into gloves and other articles. 



Vernacular Name. Fan-mussel. 



1. Atrina zelandica, Gray, 1835. Plate :>7. tig. 1. 



Pinna zelandica, Gray, Yate N.Z., 310 ; Dieff. N.Z., 259 ; Conch. Icon., xi, 

 f. 13 ; M.N.Z.M., 169. P. senticosa, Gould, P. Bost. S.X.H., iii, 1850, 

 312 ; U.S. Expl. Exp., xii, 448, atlas f. 574. 



Shell large, wedge-shaped, rather firm, of a dusky colour, with 

 longitudinal scaly ribs. Beaks terminal, approximate, sharply rounded. 

 Anterior end pointed, the dorsal margin slightly ascending and convex 

 to straight ; the basal margin first horizontal, then descending slightly 

 and straight to the posterior end, which is slightly oblique and obtusely 

 rounded. Sculpture consisting of rather close obsolete longitudinal 

 ribs, absent on the lower anterior part of the valves, armed with close 

 short semicylindrical hollow- spines, mostly lost on the anterior part 

 of the valves ; close lines of growth are prominent on the lower part, 

 where the longitudinal ribs are wanting. Epidermis horny, thick, 

 very brittle. Colour brown, inclined to purple. Interior light brown 

 posteriorly, the anterior part bluish and purple, slightly iridescent 

 and shining. Hinge-line long, very narrow. Ligament narrow, in- 

 ternal, extending nearly the whole length of the valves. Anterior 

 adductor -scar behind the beak, round ; posterior adductor -scar sub- 

 central. 



Length, 226 mm. ; height, 125 mm. ; diameter, 48 mm. 



Type in the British Museum. 



Hob. Throughout New Zealand, in mud and sand, from low-water 

 mark to a depth of several fathoms. 



Fossil in the Eocene. Miocene, and Pliocene. 



SUBORDER 2. SUBMYTILACEA. 



Eulamellibranchia in which the mantle is only slightly closed ; 

 generally there is only a single suture. Siphons absent or very short. 

 Gills smooth. Nearly always dimyarian. 



Shell equivalve, with an external ligament. 



