846 PELECYPODA. [Filibranchia. 



posterior and lower side of it is the very slightly impressed adductor- 

 scar. Byssiferous plug calcined. 



Left valve : Length, 67 mm. ; height, 55 mm. (type). Length, 

 !)5 mm. ; height, 95 mm. ; diameter, 16 mm. 



Type in the British Museum. 



Hob. New Zealand (Dr. Stanger). Hauraki Gulf ; Wellington ; 

 Pelorus Sound ; Picton ; Queen Charlotte Sound, in 16 fathoms ; 

 French Pass, in 12 fathoms (Captain Bollons) ; off Lyttelton, in 100 

 fathoms ; off Shag Point, in 30 fathoms ; off Cape Saunders, in 100 

 fathoms ; off Long Point, in 120 fathoms (E. R. Waite) ; Dusky 

 Sound (A. Hamilton) ; Foveaux Strait ; Stewart Island ; Snares and 

 Bounty Islands, in 50 fathoms (Captain Bollons). Also Australia. 



Remarks. -I have never been able to distinguish more than one 

 species amongst the considerable number of New Zealand shells which 

 I have examined, and comparison of specimens of the Australian P. ione 

 with the shells found on our shores has confirmed my view that the 

 later species of Gray, P. ione, is a synonym of his P. zelandica. 

 Mutton's A. Stowei is absolutely identical with P. zelandica, as is 

 evidenced by the diagnosis and the type specimen, which is in the 

 Dominion Museum, and which I have examined. 



Fossil in the Pliocene and Miocene ; also in the Miocene of Aus- 

 tralia. 



SUBORDER 2. ARCACEA. 



Symmetrical animals, with the mantle open throughout its extent, 

 and with generally well-developed anterior and posterior adductor 

 muscles ; the pallial line simple, without siphons ; foot variable, 

 deeply grooved, byssogenous. The heart lies in the pericardium, and 

 gives off 2 aortse. The gills are free and without interlamellar junc- 

 tions. The renal and genital orifices are separate. 



Shell of varied form, usually with a pilose epidermis, porcellanous, 

 with tubuliferous non-prismatic external layer ; area typically amphi- 

 detic ; ligament external, ali- or multi-vincular. 



Marine or fluviatile. 



Fam. ARCID-ffi, Dall. 



Animal having a stout, short, deeply grooved foot, very often 

 with a byssus ; 2 adductor muscles of about equal strength. The 

 borders of the mantle bear compound pallial eyes. The labial palps 

 are direct continuations of the lips. 



Shell trapezoidal or rounded, with the posterior side longer ; liga- 

 ment usually multivincular ; hinge typically taxodont, with the teeth 

 in two similar series, meeting below the beaks, and approximately 

 vertical to the margin of the valve. 



Jura to Recent. 



