624 



CO MI' i:\DH~M OF GENERIC DISTINCTIONS. 



Genus. Malleus. — Shell, when adult, T-shaped. China, 

 Australia. 



Genus. 1'inna. — Valves equal, wedge-shaped, posterior on 

 side gaping; binge toothless. Edge of mantle with double fringe; 

 byssus very large and powerful; gills lung. Many .seas. 



Family. MYTILIDS;.— Valves equal, oval, or lung, close 

 throughout; epidermis thick and dark. Animal spins a byssus; 

 two gills mi each side; feet cylindrical and grooved. 



Genus. Myth. is. — Shell pearl-lined, rounded behind, wedge- 

 shaped; hinge with very small and shallow teeth. Edges of 

 mantle slightly projecting, edges of gills fringed, byssus thick. 

 World-wide. 



Genus. Modiola. — Shell oblong, widened in front; hinge 

 toothless; epidermis often deeply fringed. Most seas. 



Genus. Lithodomus. — Shell cylindrical, wedge-shaped be- 

 hind. .Many seas. 



Genus. Hukissena. — Shell like Mytilus, hut nut pearl-lined; 

 a blunt keel to each valve. Many seas. 



Family. ARCADJE.— Shell regular; epidermis thick; a long 

 series of teeth on hinge. Mantle open; foot large and hunt; 

 gills very oblique. 



Genus. Aeca. — Valves nearly equal, deeply-ribbed; hinge 

 straight, with many transverse teeth. Foot long, pointed, and 

 deeply grooved; oeelli on mantle; gills lung and narrow. 

 World-wide. 



Family. UNIONIDa:.— Valves equal, closed, pearly; epider- 

 mis thick. Front hinge-teeth thick and grooved. Edges of 

 mantle united between openings of siphons; branchial orifice 

 fringed. Foot very large; gills long. 



Genus. Uxio. — Shell oval or long, thickening with age; 

 hinge teeth rather deep. Edges of mantle only united between 

 orifices of siphons. World-wide. 



Genus. Dii'S.vs. — Valves produced into a thin elastic wing. 

 Warm rivers. 



Genus. Anodun. — Shell like Unio, hut without teeth, oval 

 and smooth. Northern Hemisphere. 



Sub-order. SIPHONIDA— With respiratory siphons; lobes 



of mantle united. 



a. Siphons short; pallial line simple. 



Family. CHAMIDJE.— Shell thick, attached; two hinge-teeth 

 in one valve, one in the other. Mantle closed; orifices of siphons, 

 anil foot, small. 



Genus. Cham \. —Valves foliated, upper the smaller; hinge- 

 tooth of free valve between two of other. Two rows of filaments 

 on edges of mantle; orifices of siphons wide apart; gills deeply 

 plaited. Hot seas. 



Family. TRIDACNIDJE.— Shell blunt in front, very deeply 



Lobes of mantle joined behind, and pierced fur the orifices of 

 the siphons. Foot thick; two large gills tin each side. 



Genus. IsoCABDIA. — Shell heart-shaped; umbones spiral. 

 Foot triangular; openings of siphons close together. Must parts 

 of world. 



/i. Siphons lung; pallial line waved. 



Family. VENERIDJE.— Shell regular, close; ligament exter- 

 nal. Animal locomotive; siphons unequal, united; foot tongue- 

 shaped; gills large and squared. 



Genus. Venus. — Shell thick, edges indented; three hinge- 

 teeth in each valve. Edges of mantle fringed; siphons partly 

 separated. World-wide. 



Genus. Cytherea. — Shell like Venus, edges nut indented. 

 Edges of mantle plain; siphons half united. World-wide. 



Family. MACTRIDJE. -Shell three-sided, close or slightly 

 gaping; ligament internal; epidermis thick; two diverging teeth 

 on hinge. Mantle open in front; foot compressed. 



Genus. MaOTRA, — Shell nearly equal-sided; front hinge-tooth 

 angular. Mantle open to siphons, edges fringed; outer gills 

 shortest. World-wide. 



Family. TELLINID^l. — Shell compressed; not more than two 

 hinge-teeth; ligament on shorter side of shell. Mantle widely 

 open in front; siphons separated, very long; gills unequal. 



Genus. Scrobicularia. — Shell oval, thin; hinge-teeth small. 

 Edges of mantle toothed; siphons slender; orifices simple. 

 Many seas. 



Family. SOLENIDJE. - Shell long, gaping at both ends. Foot 

 very large and powerful; siphons short and united; gills narrow, 

 projecting into branchial siphon. 



Genus. Solen, — Shell very long, nearly cylindrical, nearly or 

 quite straight, edge parallel; two hinge-teeth in each valve; 

 ligaments external, long. Mantle closed except at front end; 

 foot blunt and cylindrical. All except frozen seas. 



Family. MYACIDJE — Shell thick, gaping behind; epidermis 

 wrinkled. Mantle nearly closed; font small; siphons united, 

 retractile; two lung gills on each side. 



Genus. Mya. — Shell oblong, left valves swollen, with a flat- 

 tened gristly process. Foot straight ; siphons fringed at orifices; 

 an inner series of fringes in branchial orifice. Gills uot project- 

 ing into siphon. Northern Hemisphere. 



Family. GASTROCILENIDJE. - Shell gaping; valves thin, 



toothless, sometimes cemented to a shelly tube. Siphons very 



long, contractile; orifices fringed. Edges of mantle very thick 



| in front and united ; gills narrow, projecting into branchial 



siphon. 



Genus. Gastroch^ena. — Shell wedge-shaped, close behind, 

 open in front ; ligament external. Foot grooved ; siphons united 

 nearly to tips. Most seas. 



ribbed; edges toothed. Animal with byssus, or free; orifices of ! Genus. Aspergillus. — Shell smooth, cemented to end of 



siphons with thick border; two narrow deeply-plaited gills on shelly tube; only umbones visible; tube closed by perforated 



eaeli side. I disc. Mantle fringed in trout. Warm seas. 



limn*. Tkiiiacna.— Shell very thick, three-sided, deeply j Family. PHOLADIDiE.— Shell open at both ends, thin, hard, 



ribbed and foliated; edges deeply toothed; byssal opening large. I brittle, and marked like a file externally ; hinge-plate reflected 



Hut seas. over umbones. Animal worm-like ; foot short; mouth closed in 



Genus. Hippopus. — Valves close, two hinge-teeth in eacl 

 byssus small. Coral st-as, on reefs. 



Family. CAR HADJE. — Valves equal, free, heart-shaped, 

 ribbed; hinge-teeth two. Mantle open in front; orifices of si- 

 phons fringed. Two thick gills mi each side; foot large and 

 curved. 



Genus. Cardutm. -Shell swollen, sometimes gaping behind; 

 edges slightly notched. Edges of mouth plaited; foot long. 

 Northern Hemisphere. 



Familij. CYPRINIDJE. -Shell rogiila 



va Ives el 



pial and (dose; 



front; siphons long, large, united nearly to tips; orifices fringed ; 

 gills narrow, projecting into exhalent siphon. 



Genus. Pholas, — Shell nearly cylindrical, accessory valves. 

 Foot large; body with fin-like termination. 



Oeinif!. Teredo.— Shell globular, open at both ends; valves 

 three-luhi-d, with a transverse furrow. Foot-opening very small : 

 viscera within valves ; gills long, passing into siphon. Siphons 

 with two shelly styles, orifices fringed. Most seas. 



Onlrr. TUNICATA. -Head uot distinct, inclosed ill leathery 

 or membranous sac, and finished with two apertures. Second 



I'pidermis thick; ligaments external. Hinge-teeth in both valves. | sac within the former, also perforated. Gills internal. 



