Along the shores of the Channel and in the Mediterranean there are few bivalves more abundant than the several species of the genus _Donax_. They live near the shore in shallow water, burying themselves perpendicularly in the sand. They have the very singular habit, considering their apparent helplessness, of being able to leap to a certain height and then project themselves ten or twelve inches. This may often be witnessed in the case of individuals left by the retreating tide. If seized by the hand, and attempts are made to disengage them from the sand, they continue to impress on their shell a sudden and energetic movement, aided by the elasticity of their foot, which is at once decisive and angular. [Illustration: Fig. 136. Donax rugosus (Linnæus).] [Illustration: Fig. 137. Donax denticulatus.] The shell of the _Donax_ is nearly triangular in shape, compressed, longer than it is high, regular, equivalve, not equilateral; the hinge with three or four teeth on each valve. The animal is slightly compressed, and more or less triangular. Its mantle, which forms two symmetrical lobes enveloping the body, is open pretty nearly in all its extent, but it is united posteriorly, and terminates in two syphons or nearly equal tubes, as in Fig. 130, p. 326. One of these tubes serves the purpose of respiration: it is the _bronchial syphon_. The other, serving the purpose of ejecting the products of digestion, is termed the _anal_ tube. The tentacles of the bronchial tube seem to be possessed of exquisite sensibility. When touched, the animal draws in its syphon, and only puts it forth anew when the danger has passed. The species of _Donax_ are very numerous, especially in the Asiatic and American seas. Among the European species we may mention _Donax rugosus_ (Fig. 136) and _Donax denticulatus_ (Fig. 137). [Illustration: Fig. 138. Tellina radiata (Linnæus).] [Illustration: Fig. 139. Tellina virgata (Linnæus).] [Illustration: Fig. 140. Tellina sulphurea (Lamarck).] [Illustration: Fig. 141. Tellina donacina (Linnæus).] Next to _Donax_ naturalists rank the genus _Tellina_, which includes many species of very minute shells, all remarkable for their beauty of form, and for their brilliant and varied colours. One of these, called the Rising Sun (_Tellina radiata_), is represented in Fig. 138. The Tellinas are found in every sea; the French coast furnishes many species: examples, _Tellina virgata_ (Fig. 139) and _Tellina sulphurea_ (Lamarck) (Fig. 140). In Fig. 141 _Tellina donacina_ is represented with its two vital tubes, or syphons. The seventh family, or Mactridæ, include Lutraria and Mactra, or the otter and kneading-trough shells. They are widely distributed; there are several British species of both.