Our seventeenth family are the Trigoniadæ, affording Trigonia, of which so many occurred in the Jurassic period of Geologic History in the strata of Europe, but of which two or three are alone left alive in the seas of Australia.
The eighteenth family, the Arcadæ, affords between 200 and 300 species of the families of Leda, Nucula, Pectunculus, and Arca.
Of the eighteenth family, Arcadæ, we shall only at present instance Pectunculus.
Fig. 153. Pectunculus aureflua (Reeve).
Fig. 154. Pectunculus delessertii (Reeve).
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).