500 MAXIJAL OF THE MOLirSCA. 



surrounded by separate areas ; sij)lions distinct, unequal, small, 

 slightly fringed ; a minute fourth orifice close to the base of the 

 branchial siphon ; visceral mass large, foot small and conical ; 

 mouth rather large, upper lip hood-like ; palpi tapering, few- 

 plaited ; gills one on each side, triangular, plaited, divided by 

 an oblique line into two portions ; excurrent channels four, two 

 at the base of the gills and two below the dorsal laminae. 

 (Hancock, An. Nat. Hist., 1853.) 



Distribution, 5 species. New South "Wales ; attached to Crass- 

 atella and Trigonia, in 8 fathoms water ; the fry (as indicated 

 by the umbones) is free, regular, and Myadora-shaped. 



Chamostrea, Eoissy. 



Type, C. albida, PI. XXIII., Fig. 14. 



Synonym, Cleidothserus, Stutch. 



Shell inequivalve, chama-shaped, solid, attached by the 

 anterior side of the deep and strongly-keeled dextral valve ; 

 umbones anterior, sub-spiral ; left valve flat, with a conical 

 tooth in front of the cartilage ; cartilage internal, with an oblong, 

 curved ossicle ; muscular impressions large and rugose, the 

 anterior very long and narrow ; pallial line simple. 



Animal with mantle-lobes united by their extreme edge 

 between the pedal orifice and siphons ; pedal opening small, 

 with a minute ventral orifice behind it ; siphons a little apart, 

 very short, denticulated ; body oval, terminating in a small, 

 compressed foot ; lips bilobed, palpi disunited, rather long and 

 obtusely pointed ; gills one on each side, large, oval, deeply 

 plaited, prolonged in front between the palpi, united posteriorly; 

 each gill traversed by an oblique furrow, the dorsal portion con- 

 sisting of a single lamina with a free margin. (Hancock, An. 

 Nat. Hist., Feb., 1853.) 



Distrihution, 1 species. New South Wales. 



Family XX. — Gastroch^nidvE. 



Shell equivalve, gaping; valves thin, edentulous, united by 

 a ligament, sometimes cemented to a shelly tube when adult ; 

 adductor impressions 2, pallial line sinuated. 



Animal elongated, truncated in front, produced behind into 

 two very long, united, contractile siphons, with cirrated orifices; 

 mantle-margins very thick in front, united, leaving a small 

 opening for the finger-like root ; gills narrow, prolonged into 

 the branchial siphon. 



The shell-fish of this family, the tuhicoUdce of Lamarck, are 



