COMPENDIUM OF GENERIC DISTINCTIONS. 



787 



tienux. CLEODORA. Shell pyramidal, triangular, tip 

 ijharp. Eyes and tentacles scarcely visible. Warm and hot 



sous. 



Genus. THECA. Shell straight, conical, pointed ; aper- 

 ture triangular. Temperate and warm seas. 



Genus. CYMBOLIA. Shell soft, slipper-shaped, sharp in 

 front, blunt behind. Fins large and rounded. Warm and 

 hot seas. 



Order. BRACHIOPODA. Bivalve molluscs, valves 

 unequal ; mostly toothed. 



Family. TEREBRATULID-ffi! Shell covered with 

 minute points ; lower valve with large beak and two hinge- 

 teeth. Animal attached either by footstalk or by ventral 

 valve. JJorsal valve with hinge and loop. 



Genus. TEREBRATULA. Shell smooth, convex ; beak 

 perforated and blunt ; loop short and attached to hinge- 

 plate. Animal attached- by footstalk. Mediterranean. 



Family. RHYNCHONELLID-ffi. Shell without the 

 points, beaked ; opening behind beak. Hinge-plate deeply 

 divided. 



Genus. RHYNCHONELLA. Shell three-sided, beak sharp ; 

 dorsal valve depressed at sides ; ventral vaive hollowed on 

 centre. Animal with long spiral arms directed inwards. 

 Most temperate and cold seas. 



. Family. LINGULID^. Shell obiong ; attached by foot- 

 stalks passing between valves. Animal with fringed mantle. 



Genus. LIXGULA. Shell compressed, gaping at both 

 ends ; dorsal valve rather short. Branchial veins arranged 

 in series of loops. Warm and hot seas. 



Order. CONCHIFERA Bivalves with body com- 

 pressed, covered on both sides by a flap of the mantle. 

 Gills at sides of body, under the mantle flaps. Valves con- 

 nected by an elastic ligament at hinge. 



Sub-order. ASIPHON1DJ5. No respiratory siphons. 



Family. OSTEEID^E. Valves unequal ; ligament in- 

 ternal ; adductor impression single. Animal marine ; mantle 

 open ; two crescent-shaped gills on each side. 



Genus. OSTREA. Shell attached by left valve ; hinge 

 toothless ; structure laminated. Margin of mantle double; 

 lips plain ; gills nearly equal. World-wide. 



Genus. ANOMIA. Shell thin, translucent, attached by a 

 plug passing through a hole in notch in right valve. Short 

 double fringe on mantle. World-wide. 



Genus. PLACUNA. Shell free, very flat, translucent, 

 rounded, resting on right valve. Indian seas. 



Genus. PECTEN. Shell regular, rounded, mostly ribbed. 

 Animal with a row of black ocelli and a fringe of" delicate 

 filaments. Gills crescentic and delicate. World-wide. 



Genius. HTNNITES. Shell regular, and with byssus when 

 young, cementing itself and becoming irregular when old. 

 Europe. 



Genus. LIMA. Shell oblique, oval, front side straight 

 and gaping. Edges of mantle separate, inner fringed with 

 tentacles ; ocelli very small. Europe. 



Genus. SPONDYLUS. Shell irregular, ribbed radiately, 

 rather covered with spines or leaf-like projections. Lips of 

 animal foliaceous. World-wide. 



Family. AYICTJLID./E. Valves unequal, oblique, 

 attached by byssus ; hinge-line straight and long, hinge 

 smooth or very slightly toothed. Foot spinning a byssus. 

 All in tropical and warm seas. 



Genus. AVICULA. Shell obliquely oval ; cartilage pit 

 single, oblique ; one or two small teeth on hinge. Ad- 

 ductor impression large, near centre ; pedal small and near 

 am bo. 



Genus. MALLEUS. Sue!!, when adult, T-shaped. China, 

 Australia. 



Gem s. PINNA. Valves equal, wedge-shaped, posterior 

 on side gaping; hinge toothless. Edge of mantle with 

 doubly fringe ; Dyssus very large and powerful ; gills long. 

 Many seas. 



Family. MYTILID^B. Valves equal, oval, or long, close- 

 throughout ; epidermis thick and dark. Animal spins a 

 byssus ; two gills on each side ; foot cylindrical and grooved. 



Genus. MYTILUS. 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 

 behind. Many seas. 



Genus. DIIEISSENA. Shell like Mytilus, but not pearl- 

 lined ; a blunt keel to each valve. Many seas. 



Family. ARCAD.ZE. Shell regular; epidermis thick; a 

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

 bent ; gills very oblique. 



Genus. ARCA. Valves nearly equal, deeply-ribbed ; hinge 

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

 and deeply grooved ; ocelli on mantle ; gills long and narrow. 

 World-wide. 



Family. TJNIONID2E. Valves equal, closed, pearly ; epi- 

 dermis thick. Front hinge-teeth thick and grooved. Edges 

 of mantle united between openings of siphons ; branchial 

 orifice fringed. Foot very large ; gills long. 



Genus. UNIO. Shell oval or long, thickening with age ; 

 hinge-teeth rather deep. Edges of mantle only united be- 

 tween orifices of siphons. World-wide. 



Genus. DIPSAS. Valves produced into a thin elastic 

 wing. Warm rivers. 



Genus. ANODON. Shell like Unio, but without teeth, 

 oval and smooth. Northern Hemisphere. 



Sub-order. SIPHONIDA. With respiratory siphons; 

 lobes of mantle united. 



a. Siphons short ; pallial line simple. 



Family. CHAMIDJK. Shell thick, attached ; two hinge- 

 teeth m one valve, one in the other. Mantle closed ; orifices 

 of siphons, and foot, small. 



Genus. CHAMA. 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. TRIDACNID.E. Shell blunt in front, very deeph 

 ribbed ; edges toothed. Animal with byssus, or free ; on 

 fices of siphons with thick border ; two narrow deeply- 

 plaited gills on each side. 



Genus. TRIDACNA. Shell very thick, three-sided, deeply 

 ribbed and foliated ; edges deeply toothed ; byssal opening 

 large. Hot seas. 



Genus. HIPPO PUS. Valves close, two hinge-teeth in 

 each ; byssus small. Coral seas, OE reefs. 



Family. CARDIADjE. Valves equal, free, heart-shaped, 

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

 siphons fringed. Two thick gills on each side ; foot large 

 and curved. 



Genus. CARDIUM. Shell swollen, sometimes gaping be- 

 hind ; edges slightly notched. Edges of mouth plaited ; 

 foot long. Northern Hemisphere. 



Family. CYPRINID.ZE. Shell regular ; valves equal 

 and close ; epidermis thick ; ligaments external. Hinge- 

 teeth in both valves. Lobes of mantle joined behind, and 

 pierced for the orifices of the siphons. Foot thick ; two 

 large gills on each side. 



Genus. ISOCARDIA. Shell heart-shaped ; umbones spiral. 

 Foot triangular ; openings of siphons close together. Mont 

 parts of world. 



. Siphons long ; pallial line waved. 



Family. VENERIDJE. Shell regular, close; Sgament 

 external. Aninril locomotive ; siphons unequal, united ; 

 toot tongue-shaped ; j\lls large and squared. 

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