270 THE LAMELLIBRANCHIA 



Scrobicularia, Schumacher ; estuarine ; British. Syiidosmya, Becluz ; 

 British. Cumingia, Sowerby. FAMILY 3. DONACIDAE, Fleming. Ex- 

 ternal gill -plate directed ventrally. Siphons separate, of moderate 

 length, the anal siphon being the longer. Foot large and compressed. 

 Shell inequilateral, the anterior side being the longer ; the ligament 

 external. Genera Donax, Linnaeus ; British. Iphigeneia, Schumacher. 

 FAMILY 4. MBSODESMATIDAE, Deshayes. External branchial plate 

 directed ventrally. Siphons separate and equal. Shell inequilateral, 

 the anterior side being the longer ; ligament internal. Genera Meso- 

 desma, Deshayes. Ervilia, Turton ; British. FAMILY 5. CARDILIIDAE, 

 Ball. Shell very high and short, ventricose, dimyarian, the posterior 

 adductor impression borne on a prominent rnyophorous apophysis. 

 Ligament partly internal. Genus Cardilia, Deshayes ; from the Pacific 

 Ocean. FAMILY 6. MACTRIDAE, Gray. External branchial plate directed 

 ventrally. Siphons united, more or less invested by a chitinous' sheath. 

 Foot long, stout, bent at an angle and without a byssus. Shell sub- 

 triangular and nearly equilateral ; the ligament partly internal. Genera 

 Mactra, Linnaeus ; British (Fig. 191). Mulinia, Gray. Harvella, Gray. 

 Baeta, Gray. Eastonia, Gray. Heterocardia, Deshayes. Vanganella, Gray. 



SUB-ORDER 4. VENERACEA. 



Eulamellibranchia with two pallial sutures ; the siphons generally 

 somewhat elongated and partially or wholly united. Gills slightly 

 folded. A bulb on the posterior aorta. Ligament external. 



FAMILY 1. VENERIDAE, Gray. Foot well developed. Adductor 

 muscles subequal. Pallial sinus shallow or absent. Genera Venus, 

 Linnaeus ; siphons rather short, their distal extremities free ; foot without 

 byssus ; British. Dosinia, Scopoli ; siphons long and fused together 

 throughout their length ; foot truncated without a byssua ; British 

 Tapes, Megerle ; siphons rather long and incompletely fused ; foot 

 byssiferous ; British (Fig. 202). Cyclina, Deshayes. Lucinopsis, Forbes 

 and Hanley ; British. Meretrix, Lamarck (Fig. 189). Circe, Schumacher ; 

 British. Venerupis, Lamarck. FAMILY 2. PETRicOLrbAE, d'Orbigny. 

 Boring Lamellibranchs with a reduced foot. The shell more or less 

 elongated, with a deep pallial sinus. Genera Petricola, Lamarck ; the 

 British species P. pholadiformis, originally an inhabitant of the United 

 States, has been acclimatised for some years in the North Sea : it has 

 boring habits as and mimics Pholas caiidida. FAMILY 3. GLAUCOMYIDAE, 

 Chenu. Siphons very long and united. Foot small. Shell elongated, 

 thin, with a deep pallial sinus. Inhabitants of fresh or brackish water. 

 Genera Glamomya, Woodward ; from S.E. Asia. Tanysiphon, Benson ; 

 from India. 



SUB-ORDER 5. CARDIACEA. 



Eulamellibranchia with two pallial sutures. Generally with short 

 siphons. The foot cylindrical, more or less elongated, furnished with a 

 byssogenous apparatus. The gills much folded. Shell equivalve, with 

 radiating costae and an external ligament. 



