COMPENDIUM OF GENERIC DISTINCTIONS. 



623 



Genus. Cylichxa. — Shell cylindrical; aperture narrow and 

 rounded in front. Animal not investing shell. Many parts of 

 world. 



Genus. APLUSTBDJl. — Shell oval, swollen; aperture blunt in 

 front; outer lip sharp. Foot capable of investing the whole 

 shell. Warm seas. 



Genus. Puilixe. — Shell oblong; aperture very wide in front. 

 Head large and oblong; eyes none; foot short and broad. Warm 

 and temperate seas. 



Family. APLYSIAD.E.— Sbell none, or small and covered by 

 mantle. Animal like slug; head distinct; foot long, forming a 

 tail behind. Very large lobes at sides. 



Genus. Aplyria. — Shell oblong, flexible, translucent. Ani- 

 mal oval; neck long; four tentacles. Sides with large lobes 

 used as tins; gill in centre of back. Most seas. 



Genus. Dolabella. — Shell hatchet-shaped. Gill near end 

 of body. Hot seas. 



Genus. Lobiger. — Shell oval, transparent; on middle of 

 back covered with epidermis. Animal slender; eyes small, on 

 sides of head. Two pairs of rounded lobes on sides. Warm 

 seas. 



Family. PLEUROBRANCHIDiE.— Shell mostly limpet-like; 

 mantle covering back; gill on sides. 



Genus. Umbrella.— Shell limpet-like, flat, round, and con- 

 centric. Foot large, deeply notched in front. Mantle small. 

 Gill on right side, forming a series of plumes. Warm and hot 

 seas. 



Sub-order. NUDIBEANCHIATA. — No shell in adult. 

 Gills always external on back or sides of body. 



Family. DORIDJE.— Oblong; gills in circle on middle of back. 

 Tentacles two. Skin with spicuhe. 



Genus. Doris;. — Oval, flattened; mantle large. Tentacles 

 retractile. All seas. 



Family. TR1TONIAD.E.— Gills along sides of back; tentacles 

 retractile. 



Genus. Dexdroxotus. — Long; tentacles laminated; branched 

 appendages in front of head. Single series of gills on each side 

 of back. Northern seas. 



Genus. DoTO. — Slender; sheathes of tentacles trumpet- 

 shaped; tentacles slender, not laminated. Northern seas. 



Family. EOLIDiE.— Gills papillose, along sides of back; ten- 

 tacles not retractile. No distinct mantle. 



Genus. Eolis. — Kather oval. Gills numerous and overlap- 

 ping; mouth with horny upper jaw. World-wide. 



Genus. Glaucus. — Long and slender. Gills on three pairs of 

 lateral lobes. Atlantic and Pacific. 



Order. NUCLEOBRANCHIATA.— Respiratory and digestive 

 organs forming a kind of nucleus on posterior part of back. 



Family. FIROLIDiE.— Long, cylindrical, translucent; ventral 

 and tail tins. Mantle with circular lip. 



Genus. Firola. — Head long and slender; fin with small 

 sucker. Tail keeled; branchial processes numerous, conical, 

 and slender. Tentacles four; eyes black and distinct. Warm 

 and hot seas. 



Genus. Carinaria. — Shell glassy, limpet-shaped, keeled. 

 Animal large, translucent; head thick and cylindrical. Warm 

 and hot seas. 



Order. PTEROPODA.— Sides towards the front furnished 

 with swimming daps. 



Sub-order. THECASOMATA.— Animal with external shell; 

 head indistinct; font and tentacles scarcely visible. 



Family. HYALEIDJE. -Shell straight, curved, globular, or 

 needle-shaped. Animal with two large tins. Body inclosed in 

 mantle. 



Genus. Hyalea. — Shell globular, translucent. Animal with 

 long appendages to mantle, passing through slits in shell. 

 Warm and hot seas. 



Genus. Cleodora. — Shell pyramidal, triangular, tip sharp. 

 Eyes and tentacles scarcely visible. Warm and hot seas. 



Genus. TheCA. — Shell straight, conical, pointed; aperture 

 triangular. Temperate and warm seas. 



Genus. CYMBULIA. — Shell soft, slipper-shaped, sharp in front, 

 blunt behind. Fins large and rounded. Warm and hot seas. 



Order. BRACHIOPODA.— liivalve molluscs, valves unequal; 

 mostly toothed. 



Family. TEREBRATULID3:. —Shell covered with minute 

 points; lower valve with large beak and two hinge-teeth. Ani- 

 mal attached either by footstalk or by ventral valve. Dorsal 

 valve with hinge and loop. 



Genus. TerebratuLA. — Shell smooth, convex; beak perfo- 

 rated and blunt; loop short and attached to hinge-plate. Ani- 

 mal attached by footstalk. Mediterranean. 



Family. RHYNCH0NELLID3:.— Shell without the points, 

 beaked; opening behind beak. Hinge-plate deeply divided. 



Genus. Riivxciionei.i.a. — Shell three-sided, beak sharp; dor- 

 sal valve depressed at sides; ventral valve hollowed on centre. 

 Animal with long spiral arms directed inwards. Most temperate 

 and cold seas. 



Family. LINGULID2E.— Shell oblong; attached by footstalks 

 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. ACEPHALA. — Bivalves with body compressed, cov- 

 ered on both sides by a flap of the mantle. Gills at sides of 

 body, under the mantle flaps. Valves connected by an elastic, 

 ligament at hinge. 



Sub-order. ASIPII0NID.E.— No respiratory siphons. 



Family. OSTREIDJE.— Valves unequal: ligament internal; 

 adductor impression single. Animal marine; mantle open; two 

 crescent-shaped gills on each side. 



Genus. Ostrea. — Shell attached by left valve; hinge tooth- 

 less; structure laminated. Margin of mantle double; lips plain; 

 gills nearly equal. World-wide 



Genus. Axouia. — Shell thin, translucent, attached by a 

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

 double fringe on mantle. World-wide. 



Genus. Placixa. — Shell free, very flat, translucent, rounded, 

 resting on right valve. Indian seas. 



Genus. Pectkx.— Shell regular, rounded, mostly ribbed. Ani- 

 mal with a row of black ocelli and a fringe of delicate filaments. 

 Gills crescentic and delicate. World-wide. 



Genus. Hixmtes.— 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 tenta- 

 cles; ocelli very small. Europe. 



Genus. Sponuvus.— Shell irregular, ribbed radiately, rather 

 covered with spines or leaf-like projections. Lips of animal fol- 

 iaceous. World-wide. 



Family. A.VICV11DX.— 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 binge. Adductor impression 

 large, near centre; pedal small and near umbo. 



