Fam. 3. Donacidae.—External gill-plate directed ventrally; siphons separate, of moderate length, anal siphon the longer. Donax; British. Iphigeneia.
Fam. 4. Mesodesmatidae.—External gill-plate directed ventrally; siphons separate and equal. Mesodesma. Ervilia; British.
| Fig. 29.—The same animal as fig. 28, with its foot and siphons expanded. Letters as in fig. 28. (From Gegenbaur.) |
Fam. 5. Cardiliidae.—Shell very high and short; dimyarian; posterior adductor impression on a prominent apophysis. Cardilia.
Fam. 6. Mactridae.—External gill-plate directed ventrally; siphons united, invested by a chitinous sheath; foot long, bent at an angle, without byssus. Mactra; British (figs. 28, 29). Mulinia. Harvella. Raeta. Eastonia. Heterocardia. Vanganella.
Sub-order IV.—Veneracea.
Two pallial sutures, siphons somewhat elongated and partially or wholly united. Gills slightly folded. A bulb on the posterior aorta. Ligament external.
Fam. 1. Veneridae.—Foot well developed; pallial sinus shallow or absent. Venus; British. Dosinia; British. Tapes; British. Cyclina. Lucinopsis; British. Meretrix. Circe; British. Venerupis.
Fam. 2. Petricolidae.—Boring forms with a reduced foot; shell elongated, with deep pallial sinus. Petricola. P. pholadiformis, originally an inhabitant of the coast of the United States, has been acclimatized for some years in the North Sea.
Fam. 3. Glaucomyidae.—Siphons very long and united; foot small; shell thin, with deep pallial sinus; fresh or brackish water. Glaucomya. Tanysiphon.