_Shell._ Lamarck asserts that the tube and shell of this genus are quite distinct. They have the shell free and detached within the sheath, and neither of the valves fixed into the partition of the tube, which is most generally testaceous, closed, and retort-shaped at the posterior extremity. Inhabits the Indian seas. Four species. Fistulana clava. F. corniformis. F. gregata. F. lagenula. 4. Genus _Septaria_. Pl. VI. _Animal._ Unknown. _Shell._ A very long testaceous tube, gradually attenuated to its upper end, and divided internally by vaulted divisions seldom complete, the extremity of which is terminated by two slender tubes without interior partitions. Specimens of this genus have been found five feet long. Found in sand on the shores of the Indian seas. Two species. Septaria arenaria. S. maculata. 5. Genus _Teredina_. Pl. VI. A fossil genus; consisting of a testaceous cylindrical sheath, the posterior extremity closed, and exhibiting the two valves of the shell it encloses; the anterior end open. Two fossil species. 6. Genus _Teredo_. Pl. VI. _Animal._ Body very much elongated, vermiform: mantle very fine, tubular, opening only at front and below for the issue of a teat-shaped foot; mouth small; very short, distinct tubes; the inferior or respiratory one somewhat larger than the superior; labial appendages short and striated; branchiæ very long, narrow, united, and prolonged throughout the extent of the tubular cavity of the mantle; only one large contractile muscle between the valves; at the point of junction of the mantle and tubes is a muscular ring, in which is implanted a pair of corneo-calcareous appendages. _Shell._ Thick, solid, very short or annular, open before and behind; valves equal, equilateral, angular and trenchant before, only touching each other by the opposite edges; no hinge; a considerable spoon-like cavity; only one feeble muscular impression. Tube more or less distinct from the substance in which the animal lives, cylindrical, straight or winding, and closing with age at the buccal extremity so as to envelope the animal and its shell; at the other end it is always open, and divided internally into two syphons by a partition in the middle. Inhabits the European seas in timber. Three species. Teredo navalis. T. palmulata. T. gigantea. FAMILY II. PHOLADARIA. Two Genera. 1. Genus _Pholas_. Pl. VI. _Animal._ Without a tubular sheath, projecting anteriorly into two united tubes, frequently surrounded by a common skin, and the posterior extremity provided with a short muscular foot, flattened at its extremity. _Shell._ Multivalve, equivalve, transverse, gaping at both ends, with various accessory pieces either on the hinge or below it. Inhabits the Indian seas and American shores. Eighteen species.