788 



COMPENDIUM OF GENERIC DISTINCTIONS. 



Genus. VENUS. Shell thick, edges indented; three 

 hinge-teeth in each valve. Edges of mantle fringed ; siphons 

 jiartly separated. World-wide. 



Genus. CYTHEREA. Shell like Venus, edges not in- 

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

 tooth angu'ar. Mantle open to siphons, edges fnnged ; 

 outer gills shortest. World-wide. 



Family. TELLTTID.ZE. 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 ; sipuons 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.ffi. Shell thick, gaping behind ; 

 epidermis wrinkled. Mantle nearly closed ; foot small ; 

 siphons united, retractile ; two long gills on each side. 



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

 flattened gristly process. Foot straight ; s:phons fringed at 

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

 not projecting into siphon. Northern Hemisphere. 



Family. GASTROCttffiNID^l. 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. GASTROCILENA. Shell wedge-shaped, close be- 

 hind, 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 per- 

 forated disc. Mantle fringed in front. Warm seas. 



Family. PHO LAD ID^B. 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. TUN 1C ATA. Head not distinct, inclosed in 

 leathery or membranous sac, and finished with two aper- 

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

 internal. 



Family. ASCIDIADJE. Animal solitary or gregario'is, 

 simple and fixed. Oviparous. 



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

 tinous ; 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. PEI.ONJEA. 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. SYNTETHYS. Animals gelatinous, sessile ; orifices 

 simple. Northern seas. 



Family. BOTRYLLIAD.S!. Animals compound ; ter.ts 

 fused together. Oviparous and gemmiferous. 



Svb-family. Botryllina. Animals united in systems 

 round a common cavity. Thorax and abdomen not distinct. 



Genus. BOTRYLLUS. Test gelatinous or gristly ; systems 

 round or star-shaped, individuals lying horizontally. Northern 

 seas. 



Family. 1TROSOMIDJE. Animal compound, free. 



Genus. PYROSOMA. Body not contractile, cylindrical, 

 hollow, open at one end ; animals grouped in whorls on 

 exterior. Warm seas. 



Family. SALPIDJE. 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. POLY2OA. 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. 



Order. INFUNDIBULATA. 



Sub-order. CHEILOSTOMATA. Aperture of cell filled 

 with thin membranous or chalky veil ; mouth crescent-shaped; 

 lip moveable. 



Family. CATENICELLIDJE. Cells connected by flexible 

 joints. 



Getnu. CATENICELLA. Cells facing same way ; branches 

 dichotomous ; polyzoary erect ; ovicells below opening of cell 

 in front. Bass's Straits, New Zealand. 



Genus. CALPFDIUM. Avicularium on each side of cells ; 

 two or more apertures all facing same way ; cells single at 

 dichotomous fork. Bass's Straits. 



Family. SALICORNIARIDJE. Cells round imagmn y 

 axis ; branches cylindrical ; polyzoary dichotomous, erect. 



Genus. SALICORNARIA. Front of cell depressed, sur- 

 rounded by ridge, forming surface of polyzoary into hexagons 

 or rhomboids. No apertures. Avicularia irregular. Euro- 

 pean and other seas. 



Family. CELLTTLARIDJE. Cells in same plane ; branches 

 linear, dichotomous, erect. 



Genus. CELLULARIA. Cells bi-triserial, oblong or rhomb- 

 oidal, contiguous, perforated behind ; no avicularium, seldom 

 vibraculum. Britain, &c. 



Genus. MENIPEA. Sessile avicularia on front of cell below 

 aperture. Britain, &c. 



Genus. SCRUPOCELLARIA. Cells rhomboidal. Sessile 

 avicularium on each cell. Aperture oval. Britain, &c. 



Genus. CANDA. No sessile avicularium ; rjst like Scrupo- 

 cellaria. Britain, &c. 



Section. Polyzoary continuous. 



Family. SCRUPARIAD^!. Cells in single series. Joints 

 not flexible. 



Genus. SCKUPARIA. Cells decumbent ; aperture oblique 

 near end. Branches from front of cell below aperture 

 Britain, &c. 



