The genus _Pectunculus_ are abundant on the shores of the Mediterranean and along the Atlantic coast. If we take up at hazard a handful of shells on any part of the French coast, one-third will consist of _Pectunculus_. They are found mixed with Cardium, Venus, Razor-fish, and Pectens. Their round and robust frame attracts much attention. They form the first of those charming infantile collections which are gathered at the mother's feet. The animal which inhabits this pretty shell is moulded on its curvature; like the shell, it is round and squat; it is furnished with a mouth, large, and thick for its size, and with double branchiæ. When the animal is taken alive, it sometimes exudes a thick mucous liquid over the shell, which has disgusted many a young collector with his capture. Among numerous species of _Pectunculus_ we note as worthy of representation: _P. aureflua_, Reeve (Fig. 153); _P. delessertii_, Reeve (Fig. 154); _P. pectiniformis_, Lamarck (Fig. 155); and _P. scriptus_, Born (Fig. 156). [Illustration: Fig. 155. Pectunculus pectiniformis (Lamarck).] [Illustration: Fig. 156. Pectunculus scriptus (Born).] FOOTNOTES: [Footnote 8: The term bivalve as constituting a class must be taken in a limited sense, for several genera, as pholas for example, have six or more valves.] CHAPTER XII. ACEPHALOUS MOLLUSCA--_continued_. MYTILIDÆ--THE MUSSEL. "Ecce inter virides jactatur mytilus algas." _Anthologia._ We shall now consider the nineteenth family or Mytilidæ, which includes Mytilus, Modiola, Lithodomus, and Dreissena. [Illustration: Fig. 157. Mytilus edulis (Linnæus).] The well-known shell of the mussel (Fig. 157, _Mytilus edulis_) is longitudinal, equivalve, and regular, pointed at the base, with capacity to attach itself by a byssus; the hinge has no teeth, but a deep furrow, in which the ligament is located. In the genus _Mytilus_ the byssus is divided to its base. In _Modiola_ it has a common corneous centre. In _Pinna_ the anus is furnished with a long angular base. In all these genera the foot is small, its retractile muscles numerous, and the byssus large. In _Lithodomus_ the byssus is rudimentary; the muscles are retractile, equal, and two pairs only. In _Unio_, _Cardium_, and _Hyria_, the foot is large and not byssiferous.