786 



COMPENDIUM OF GENERIC DISTINCTIONS. 



Genus. LITHODOMUS Shell cylindrical, wedge-shaped be- 

 hind. Many seas. 



Genus. DRBISSBNA. Shell like mytilus, but not pearl-lined ; 

 a blunt keel to each valve. Many seas. 



Family. ARCADyE. 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, wiih many transverse teeth. Foot long, pointed, and 

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

 World-wide. 



Family. UNIONID^E. Valves equal, closed, pearly; 

 epidermis 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 between 

 orifices of siphons. World-wide. 



Afints. 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. CHAMID^E. Shell thick, attached; two hinge- 

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

 of siphons, and foot, small. 



Genus. CHAMA. Valves foliated, upper the s^naller; hinge- 

 tooth of free valve between two of other. Two rows of fila- 

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

 deeply plaited. Hot seas. 



Family. TRIDACNIDJE. Shell blunt in front, very 

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

 orifices 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. Hippopus. Valves close, two hinge-teeth in each ; 

 byssus small. Coral seas, on reefs. 



Family. CARDIAD^E. 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. CYPRINIDjE. 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. Most 

 parts of world. 



B. Siphons long ; pallial line wared. 



Family. VENERID/E. Shell regular, close; ligament 

 external. 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. 



en'is. CVTHBREA. Shell like Venus, edges not indented. 

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



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

 gaping ; ligament internal ; epidermis thick ; two diverging 

 teeth on hinge. Mantle open in front ; foot compressed. 



Genus. MACTRA. Shell nearly equal-sided; front hinge- 

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

 gills shortest. World-wide. 



Family. TELLUID./E. Shell compressed ; not more 

 than two hinge-teeth ; ligament on shorter side of shell. Man- 

 tle 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. MYACID^. Shell thick, gaping behind ; epi- 

 dermis wrinkled. Mantle nearly closed ; foot small ; siphons 

 united, retractile ; two long gills on each side. 



Genus. MVA. Shell oblong, left valves swollen, with a 

 flattened gristly process. Foot straight ; siphons fringed at 

 orifices; an inner series of fringes in branchial orifice. Gills 

 not projecting into siphon. Northern Hemisphere. 



Family. GASTROCH^NID/E. 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. 



Genus. ASPERGILLUM. Shell smooth, cemented to end of 

 shelly tube ; only umbones visible ; tube closed by perforated 

 disc. Mantle fringed in front. Warm seas. 



Family. PHOLADID.E. Shell open at both ends, thin, 

 hard, brittle, and marked like a file externally ; hinge-plate 

 reflected over umbones. Animal worm-like ; foot short; mouth 

 closed in 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. 



Genus. TEREDO. Shell globular, open at both ends; 

 valves three-lobed, 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. 



Order. TUNICATA.Head not distinct, inclosed in 

 leathery or membranous sac, and finished with two apertures. 

 Second sac within the former, also perforated. Gills internal. 



Family. ASCIDIADjE. Animal solitary or gregarious, 

 simple and fixed. Oviparous. 



Genus. ASCIDIUM. Body sessile ; tests leathery or gelat- 

 inous ; branchial orifice with eight lobes, a circle of filaments 

 within ; branchial sac simple. Northern Hemisphere. 



Genus. CYNTHIA. Test leathery, sessile ; orifices with four 

 lobes ; branchial sac plaited, tipped with filaments. Northern 

 Hemisphere. 



Genus. PBLON^EA. Body long, cylindrical; orifices four- 

 cleft ; mantle adhering to test. Northern Hemisphere. 



Genus. BOLTENIA. Test leathery, globular, on foot-stalk ; 

 orifices at sides four-cleft ; branchial sac plaited. Many seas. 



Family. CLAVELLIN^E. Animals compound and fixed, 

 connected by tunic or gelatinous base. 



Genus. CLAVELLINA. Body long, erect, on footstalk ; test 

 transparent and smooth ; orifices simple. Many seas. 



Genus. SVNTETHYS. Animals gelatinous, sessile ; orifices 

 simple. Northern seas. 



Family. BOTRYLLIADjE. Animals compound ; tests 

 fused together. Oviparous and gemmiferous. 



Sub-family. BOTRYLLINA. Animals united in systems 

 round a common cavity. Thorax and abdomen not distinct. 



Genus. BOTRVLLUS. Test gelatinous or gristly ; systems 

 round or star-shaped, individuals lying horizontally. Northern 

 seas. 



Family. PYROSOMID./E. Animal compound, free. 



Genus. PYROSOMA. Body not contractile, cylindrical, hol- 

 low, open at one end ; animals grouped in whorls on exterior. 

 Warm seas. 



Family. SALPID^E. Animals free, alternately solitary 

 and aggregated. 



Genus. SALPA. Animal oblong, nearly cylindrical. Test 

 thin, transparent ; gill rudimentary. Alternate generations 

 unlike each other. Northern Hemisphere. 



Sub-class. POLYZOA. Animals compound ; digestive 

 canal with two apertures, replicate, the posterior ascending by 

 side of front portion. Mouth surrounded with tentacles. 

 Front portion retractile by inversion. 



