Hoclistctteria.] PELECYPODA. 861 



lines. Colour pale fuscous or reddish. Interior smooth, shining, faintly 

 rayed. Hinge with 1-2 anterior and 4-5 posterior marginal teeth. 

 Ligament in a triangular resilifer, which is wide and extended on both 

 sides. Margins on the ventral and posterior sides distinctly crenate, 

 the denatures resembling the marginal teeth. 



Diameter Ant. -post., 1-5 mm. ; dorso-ventral, 1-1 mm. : thick- 

 ness, 1 mm. : prodissoconch, 04 mm. (type). 



Type in the Mus. Hist. Nat., Paris. 



Hab. Stewart Island, in 35 fathoms, type (Filhol) ; near the 

 Bounty and Snares Islands, in 50 fathoms (Captain Bollons) ; Shag 

 Point and Otago Peninsula (Iredale) ; Foveaux Strait, in 15 fathoms ; 

 Taumaki Island, in 10 fathoms (Captain Bollons) , Lyttelton Harbour, 

 ou seaweeds (H. S.) ; Lyall Bay. 



SUBORDER 3. MYTILACEA. 



Symmetrical Pelecypoda in which the anterior adductor muscle is 

 always less developed than the posterior (the " anisomyarian " con- 

 dition) or is absent. The heart gives oft" a single vessel only, the 

 anterior aorta. The gills are smooth, the gill-filaments all alike and 

 provided with interlamellar junctions. The gonads externally extend 

 into the mantle, and open at the sides of the kidneys. The foot is 

 linguilorm and byssiferous. 



Shell usually equivalve, not alate or notched for a byssus, edentulous 

 or with teeth evidently derived from external sculpture impinging upon 

 the hinge-line (dysodont). Shell- substance subnacreous, rarely more 

 or less prismatic. Epidermis conspicuous. Ligament parivincular, 

 usually opisthodetic and external. 



Mostly marine. 



Fam. MYTILID^E, Fleming. 



Animal having the mantle-lobes united below the anal siphon, 

 otherwise free ; generally byssiferous. Cephalic eyes present. 



Shell inequilateral, the anterior side being short, equivalve, hetero- 

 myarian, slightly gaping, typically dysodont ; area amphidetic or 

 none ; ligament usually external, deep-seated ; rarely with an alivin- 

 cular internal resilium ; pallial line simple. 



Devonian to Recent. 



KEY TO GENERA. 



A. Shell oblong, cuneiform. 



a. Beaks terminal, usually with teeth at the beaks . . MYTILTJS. 



b. Beaks not terminal, no teeth at the beaks . . . . MODIOLTJS. 



B. Shell rhomboidal, with 3 areas, the median usually smooth, the 



others ribbed . . . . . . . . . . MODIOLARIA. 



C. Shell oblong, subcylindrical ; hinge- plate very narrow and 



smooth . . . . . . . . . . . . LITHOPHAGA. 



D. Shell oval, trapezoidal, higher than long ; hinge-plate plicated 



on both sides . . DACRYDIUM. 



