366 



Tridacnid^.- 



-MOLLUSCA,- 



-PlIOLADISA. 



Bait water fur a time, it has probably been carried across 

 seas attached to the bottoms of ships, and in this manner 

 it appears to have reached England, and has become 

 BO common in our canals as to be much more abundant 

 than many of our indigenous mollusks. James de 

 Carle Sowerby first described it as British. This was 

 in 1824, and he stated that it was found in abundance 

 attached to shells and timber in the Commercial Docks, 

 and that the animal was used as bait for perch. Mr. 

 Stark found it in the Union Canal, near Edinburgh, in 

 1834; and the Rev. J. Berkely observed it in the river 

 Nen in 1836, though he believed it had been introduced 

 from Wisbeach, on timber, in 1828. Thus it would 

 appear to have found its way into Britain at several 

 points, and is now common in many parts of the country; 

 in London it has been found in large quantities in iron 

 water pipes, encrusted with a ferruginous deposit. On 

 the Continent it is found in the Wolga, Danube, Elbe, 

 and many of the rivers of Germany and Belgium. 



Passing over the genus Galeomma, we come to the 

 Clam Shells. 



Family— TRIDACNID^. 



Tlie Clam Shells are solid, opaque, white shells, 

 regularly equivalve, truncate anteriorly. The valves 

 are strongly ribbed externally, and the margins dentate 

 internally. The ligament is external, and the hinge is 

 furnished with very oblique, interlocking teeth. The 

 Clam Shells are the giants among the Bivalves, and 

 live attached by their byssus to rocks, shells, and corals. 

 "They also have the faculty of forming holes in the 

 surface of tlie shell or coral to which they may happen 

 to he fixed." — [Gray.) 



Genus Tridacna. — This genus has a massive 

 trigonal shell, ornamented with radiating ribs and 

 imbricating foliations. The valves have a wide gape 

 close to the beak in front, occupying nearly the whole 

 anterior side, for the passage of the large byssus. Tlie 

 hinge is composed of a single cardinal, compressed. 



tooth in each valve, two posterior lateral teeth in one, 

 and a single one in the other. 



THE GIANT CLAM SHELL [Tridacna gifjas) is very 

 large, with great imbricated squamous ribs. Its size and 

 weight, combined with the beautiful marble-like appear- 

 ance of its interior, have always caused it to be sought 

 after as an ornament for grotto work or for garden 

 fountains; and, indeed, as Mr. Broderip remarks, the 

 valve of a large individual forms a very picturesque 

 basin for catching the clear falling water, and transmit- 

 ting it through the deep interstices of its indented edge 

 to the reservoir below. In Roman Catholic countries, 

 the valves of the huge shell are sometimes used as 

 "benitiers," or vessels for containing holy water. A 

 pair, so used, may be seen in tlie church of Saint Sulpice, 

 in Paris; they weigh five hundred poun<ls, are more than 

 two feet across, and were presented to Francis I. by the 

 Republic of Venice. Specimens attain even larger dimen- 

 sions than these, and indeed are, without exception, the 

 largest shells known. The byssus is so large, and its 

 attachment to the rock so strong, that it is frequently 

 necessary to cut it with a hatchet, in order to remove 

 the shell. The animal is good eating, and some of our 

 earlier voyagers have mentioned it as furnishing their 

 sailors a wholesome meal. Captain Cook states that 

 they sometimes weigh upwards of twenty pounds; and, 

 as Mr. Broderip observes, "a fine well fed Indian giant 

 Tridacna would furnish forth a dish for a dozen." The 

 genus is figured in Plate 10, fig. 8, by Tridacna crocea. 



Genus Hippoi'US, Horse-hoof shell, or Bear's-paw 

 Clam. — This genus has a transversely ovate shell of 

 moderate size, ribbed, and squamous externally. It is 

 readily distinguished from the preceding by its valves 

 being closed and flattened in front, and, in consequence, 

 having no aperture for a byssus, and by having only 

 two compressed cardinal teeth in each valve. The 

 only known species, //. mactdatus, is a handsome shell, 

 white, spotted with red or purple, and is found on the 

 coral reefs of the Indian Ocean. The shells are now 

 much used in the ornamental arts, for making into ink- 

 stands and other useful purposes. 



Order II. PIIOLADACEA. 



In this order the animals have in general a closed 

 mantle, with two more or less elongate siphonal tubes, 

 w^hich are almost always united. Tlie gills are pro- 

 duced behind into the inhalent or lower siphon; and 

 the foot is frequently more or less elongated and club- 

 shaped. 



Family— PHOLADID^. 



The Pholades have a thin, white, brittle, and exceed- 

 ingly hard shell, witli the valves united only by a very 

 thin ligament, gaping at both ends, and armed with 

 rasp-like imbrications in front. The shells are either 

 free or within a shelly tube. The hinge is without any 

 cartilage, and has no teeth, but is often strengthened 

 externally by accessory valves. The hinge plate is 

 reflexed over the beaks, and is furnished with a long, 

 curved muscular process beneath each. The pallial sinus 



is very deep. The animal is symmetrical, club-shaped, 

 or worm-like; the mantle is closed in front, except the 

 orifice for the passage of the foot, which is club-shaped 

 and truncated ; and the siphons are large, elongated, 

 and united nearly to the ends. The Pholades are found 

 in most parts of the globe, and all (he members of the 

 tribe are borers into stone, clay, wood, or other sub- 

 stances. The species are numerous, are widely dis- 

 tributed, and they may even be said to be cosmopolitan. 

 The question how the boring mollusca excavate their 

 dwelling-places has been the subject of much discussion, 

 and is still in a great measure undecided amongst 

 Naturalists. 



PllOLADINA. — The true Pholades have an elongated 

 cylindrical shell, with the anterior gape either open 

 or, in the adult, closed by a callous plate, and tlio 

 dorsal margin covered by accessory valves. The 



