MOLLUSCS. 



273 



Fig. 290. — Tridacna mutica. 



is stated that the soft parts of Tridacna gigas will sometimes weigh twenty pounds. 

 The shells of this same species will sometimes, together, weigh five hundred pounds, 

 and in some of the churches of France they are 

 employed to hold the holy-water, a use which 

 well accords with the beautiful white of the in- 

 ner surface of the shell. This species and the 

 smaller T. squamosa are very common in col- 

 lections, the first having the ribs of the valves 

 nearly smooth, the other having them orna- 

 mented by large foliaceous processes. Both 

 these species are of considerable importance to 

 the South Sea Islanders, as they afford an im- 

 portant part of the food supply. The giant clam 

 is also useful from a mechanical point of view. 

 In many of these islands, stones are unknown, 

 but, as a substitute, the natives make their knives 

 and axes from the fragments of this shell. In 

 one species the gills are bright blue. 



Hippopus^ which differs but slightly from 

 Tridacna, has a similar distribution, and the 

 common species, H. maculatus, is found in almost every collection. The ribs on 

 the outside are much smaller and more numerous than in the other genus, and scat- 

 tered over the outside are numerous little, scale-like projections. The color, both 

 inside and out, is a delicate, creamy white, ornamented on the exterior with small 

 blotches of lake. The generic name means horse-foot, but the common name is the 

 bear's-paw clam. The apphcability of either is not very evident. 



The name Cardiid^ is applied to the next family which we have to consider on 

 account of the heart-shaped outline which the species present when viewed from the end. 

 The valves are equal and swollen out, and the umbones are rolled inward, so that the 

 appearance is like the conventional heart, which, it may be remarked, resembles no 

 heart existing in nature. The hinge ligament is external, and the hinge teeth are two 

 in each valve beneath the umbones, and one in each on either side of the central ones. 

 The external surface of the shell is usually radiately ribbed, the ornamentation on the 

 posterior part differing from that in front. The edges of the mantle are united, so 

 that a small slit is left for the protrusion of the strong, sickle-shaped foot, which plays 

 a very important part in the locomotion of these molluscs. The siphons are usually 

 very small, but occasionally they are better developed, and at such times there is a 

 slight indication of a sinus in the pallial line. 



Oardiitm, with its various subdivisions, embracing about a hundred and fifty species, 

 is found in all the seas of the world, living in shallow water, from low tide down to a 

 hundred and fifty fathoms. The shell is strongly ribbed and very swollen. Our 

 largest species is Cardium islandicum which is found north of Cape Cod. Other 

 smaller species are found on our coasts, one of which ( C. pinmdatmn) occurs as far 

 south as Long Island Sound. These shells are known as cockles, and one species is 

 eaten in England. They prefer sandy bays near low water, and are usually found 

 in large numbers together. 



In Zimvicardium, as the name indicates, the surface is nearly smooth,' and our 

 single species ranges from the West Indies to Nova Scotia, being common in the watery 



VOL. I. — 18 



