344 THE OCEAN WORLD. 
The thirteenth family of our list; Tridacnidee, contains the genus 
Tridacna, with only eight species, but it contains the largest of all 
bivalve shells. 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 Zridacna, 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 500 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 
Fig. 147.—Cardium cestatum (Linnzeus). 
presented to the church of Saint Sulpice, where they are used as 
basins for holy water. Another pair are exhibited in the church of 
Saint Eulala, at Montpelier, but much smaller in size. The shells of 
Tridacna gigas are formed, as represented in PLaTe XIV., of three 
acute angles, festooned on their edges by broad sides bristling with 
deep white scales. The hinges have two teeth; the ligament is 
elongated and external. 
The animal of Z?idacna is remarkable for its fime colours: 
Tridacna serrifera is of a beautiful blue round the edges of the mantle, 
rayed through a shade of very pale blue ; more in the interior of the 
mantle is a row of small ocelli 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, 
