270 THE LAMELLIBRANCHIA 
Scrobicularia, Schumacher; estuarine; British. Syndosmya, Recluz ; 
British. Cumingia, Sowerby. Famity 3. Donacipar, 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—Donaz, Linnaeus; British. _ [phigeneia, Schumacher. 
Fammy 4. Mesopesmatipar, 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. Faminy 5. CARDILIIDAE, 
Dall. Shell very high and short, ventricose, dimyarian, the posterior 
adductor impression borne on a prominent myophorous apophysis. 
Ligament partly internal. Genus—Curdilia, Deshayes ; from the Pacific 
Ocean. Famrty 6. Macrripar, 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. 
Raeta, Gray. Eastonia, Gray. Heterocardia, Deshayes. Vanganella, Gray. 
Sup-OrDER 4. VENERACBA. 
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. 
Famity 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 byssus; British 
Tapes, Megerle; siphons rather” long and incompletely fused ; foot 
byssiferous ; British (Fig. 202). Cyclina, Deshayes. Lucinopsis, Forbes 
and Hanley ; British. Meretriz, Lamarck (Fig. 189). Circe, Schumacher ; 
British. Venerwpis, Lamarck. Famity 2.~ Perriconipar, 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 candida. FamiIny 3, GLAUCOMYIDAB, 
Chenu. Siphons very long and united. Foot small. Shell elongated, 
thin, with a deep pallial sinus. Inhabitants of fresh or brackish water, 
Genera—Glaucomya, Woodward ; from 8.E. Asia. Tanystphon, Benson ; 
from India. 
Sus-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. 
