_Cardium costatum_ (Fig. 148) is an exotic species which inhabits the coast of Guinea and the Senegal, the shell of which, white and fragile, is much sought after by collectors. The thirteenth family of our table, Tridacnidæ, consists of only eight or ten species, but it contains the largest of all, the giant Tridacna. The historian of the wars of Alexander the Great speaks of oysters inhabiting the Indian Ocean which were more than a foot long; these were probably _Tridacna_, the shells of which were most likely to be seen by the Macedonian conquerors. The valves of _Tridacna gigas_ are sometimes found a yard and a half in length, and weighing five hundred pounds. Magnificent examples may be seen in the church of Saint Sulpice, Paris, where they hold the holy water. These beautiful shells were the gift of the Venetian Republic to Francis I. Under Louis XIV., the curé Languet had them presented to the church of Saint Sulpice, where they are used as fonts for holy water. Another pair are exhibited in the church of Saint Eulala, at Montpelier, but much smaller in size. The shells of _Tridacna_ are formed, as represented in PL. XVII., of three acute angles, festooned on their edges by broad sides bristling with white scales. The hinges have two teeth; the ligament is elongated and external. [Illustration: Plate XVII.--Tridacna gigantea, Holy Water Basin in the Church of Saint Sulpice at Paris.] The animal of _Tridacna_ is remarkable for its fine colours. _Tridacna safrana_ is of a beautiful blue round the edges, rayed through a shade of very pale blue. More in the interior is a row of small moons of a yellowish green; the centre is a bright violet, with brownish longitudinal punctured lines. "We have at this moment before our eyes," say the travellers Quoy and Gaimard, "one of the most charming spectacles that can be seen, when at a little depth beneath the surface a number of these animals display the brilliant velvety colours and varying shades of their submarine parterres. As we can only perceive the gaping opening of the valves, we may imagine to ourselves what is its first aspect." The mantle of the animal is closed and ample; its edges are swollen, and reunited in nearly its whole circumference in such a manner as to leave only three very small openings--two in the upper part; the one serves the purpose of discharging the products of digestion, the other gives entrance and exit to the water necessary for respiratory purposes. The third opening is in the lower part of the body, and free; it leaves an opening for the passage of the foot, which is enormous, and is surrounded with an ample tuft of byssoidal fibres. [Illustration: Fig. 149. Tridacna squamosa (Lamarck).] [Illustration: Fig. 150. Tridacna squamosa, on the inside (Lamarck).]