272 LOWER INVERTEBRATES. 



In all the remaining families the siphons are well developed, elongate and tubular, 

 and the edges of tlie mantle are united to a greater or less extent. In the Chamidaa, 

 Tridacnida3, Hippuritidse, Cardiidae, Lucinidae, Cycladidte, and CyprinidsE, the pallial 

 line produced by the edge of the mantle on the shell is simple, while in all the remain- 

 ing families at the posterior end of the shell it makes a re-entrant angle known as the 

 pallial sinus. This is produced by the great development of the siphonal muscles. 



The members of the Chamid^ are thick, irregular, inequivalve shells, with exter- 

 nal hinge ligament and strongly developed hinge teeth, one in one vahe and two in 

 the other. The mantle margins are united, leaving only the small siphons and a 

 small opening for the foot. The living species are found in the tropical seas of both 

 hemispheres, and all, both recent and fossil, are usually attached to some rock oi' shell 

 by one of the umbones. In Chama the shell is lamellar in texture and the umbones 

 are coiled in a spiral ; but this peculiarity reaches its greatest development in the fossil 

 genus Diceras., either valve of which, seen alone, would be taken for the shell of a 

 gasteropod until a closer examination showed the hinge, muscular impressions, etc. 



The HippuEiTiD^ (ranked as an order, Iludistes, by Lamarck) is usually placed here, 

 but its position is far from certain. The family is characteristic of the rocks of the 

 cretaceous age and in some parts of Europe have given to certain beds the name of 

 Hippurite limestone. They are also found on our continent. The shells are sui generis; 

 the two valves are very unequal, one being frequently very flat, while the other is 

 immensely drawn out, in one form resembling a cow's horn, and giving rise to the 

 name Hippurites cornu-vaccarum. The shells wei-e attached, and frequently tlie 

 larger \alve was chambered so that in foi'ms like Caprinella it bore no inconsiderable 

 resemblance to the chambered shells of the cophalopods. The position which these 

 forms should occupy in the animal kingdom has been very differently regarded by 

 different authors. Besides being placed in varying relationships among the Acejihals, 

 they have been considered as curals, barnacles, worms, and various combinations of 

 these distinct groups. The principal features which place them among the bivalve 

 molluscs are the structure of the shells, which are hinged together and furnished with 

 an internal cartilage, the presence of two adductor muscles and a well-marked jiallial 

 line. One noticeable peculiarity is found in Hipjnirites, where the free valve "is per- 

 forated by radiating canals which open around its inner margin, and communicate with 

 the upper surface by numerous pores, as if to supply the interior with filtered water." 

 In other genera there is no trace of these canals. 



The Tridacnid^, which are all inhabitants of the eastern seas, contain the largest 

 bivalves known. The shell is regular and equivalve, the valves being ornamented by 

 strong ribs radiating from the umbones. The shell is very hard, almost every trace of 

 organic matter being removed. The hinge ligament is external, and there are either 

 one or two cardinal teeth in each valve, and one posterior tooth in one, and two in the 

 other. The foot is comparatively small, finge)--like, and provided with a byssal groove. 

 Some of the species spin a byssus, but others do not. The adductor muscle is single, 

 and there are two gills on either side, the outer ones composed of but a single 

 lamina. 



The genus Tridacna embraces some very large forms, and the generic name is 

 given in allusion to this fact. The ancients used to tell of an Indian oystei' which was 

 so large that it required three bites {tri and daJcno) to devour one of them, and so, 

 when a more modern science came to name these forms, the old terms was resurrected 

 and applied as a generic name. The appropriateness of it will readily be seen when it 



