Glossophorous Mollusca, with a shell shaped like an Elephant's tusk, and open at both ends, the small as well as the large. Its concave side is dorsal, the convex ventral. The mantle, which lines the shell, arises in the larva as a right and left dorsal fold, which grow towards the ventral aspect, and fuse one with the other, except for a short distance, in front and behind. The tube thus formed extends both backwards and forwards over the animal. The shell dcvelopcs in a corresponding manner, and is at first incomplete vcntrally. The head is cylindrical, and bears the mouth at its extremity, surrounded by a circle of tentacles. Two pads, one right, the other left, at the base of the head and above the foot, give origin to a number of ciliated contractile processes, terminated by flat expansions. It is possible that they represent the ctenidia. The foot is long, and trifid at its extremity.

The nervous system consists of a pair of cerebral ganglia, united by long connectives to a pair of pedal ganglia, to which the pleural ganglia are closely apposed. The visceral loop is long, and placed below, i.e. in front of the anus. There arc no eyes, but a pair of otocysts is attached to the pedal ganglia. The digestive tract possesses a buccal mass, inclosing a radula armed with five longitudinal rows of simple teeth. The alimentary canal consists of an oesophagus, a short stomach, into which open two large symmetrically placed liver lobes, an intestine, which forms several coils and finally opens on the ventral surface in the middle line. There is no heart. The coelome consists of a series of channels and sinuses, and is filled by a colourless blood. There are two nephridial openings, one on each side the anus. The nephridial chamber is perforated by the intestine, and is beset with a number of small caeca. The genital gland lies dorsally. It is alike in both sexes, and consists of a median tube bearing three rows of caeca - one dorsal, two lateral.

The duct opens into the nephridial sac on the right side.

Segmentation is unequal, the gastrula invaginate. There are three ciliated rings surrounding the body in front of the mouth, and representing the velum. The body lengthens behind the ciliated rings. There is a rudimentary shell-gland, and perhaps a foot-gland. The anus is formed as a proctodaeum.

There are three genera, Dentalium, Siphonodentalium, and Entalium. The animal lives with the anterior extremity plunged into the sand on the sea coast, at depths of ten to a hundred fathoms. Dentalium occurs fossil in Carboniferous strata.

Dentalium, de Lacaze Duthiers, A. Sc. N. (4), vi. 1856; vii. and viii. 1857. Development, Kowalewsky, Annales Mus. Nat. Hist. Marseilles, i. 1883. Comparison with Cephalopoda, Grobben, Arb. Zool. Inst. Wien, v. 1884 (p. 44 of his paper).