ARCA ; PECTUNCULUS. CHAMACEJ2. 



409 



TIG. 694 PECTUNCULTJS. 



the world, but their remains are far more abundant in many of 



the older strata. In the 

 Pectunculus the hinge has a 

 similar elongated character, 

 but it is curved instead of 

 being straight. The Tri- 

 gonice are closely allied to the 

 Arks, but differ from them 

 in having only two or three 

 teeth in the hinge. The foot 

 is long and bent, and is said to enable them to leap with con- 

 siderable activity. The few living species are confined to the 

 Australian seas. 



ORDER IL SIPHONATA. 

 SECTION A. CHAMACE^E. 



1035. In this group are included the largest, and some of the 

 most inert, of all the Testa- 

 ceous Mollusca. Nearly all 

 of them are attached to solid 

 bodies, during the greatest 

 part of their lives ; some by 

 the adhesion of the shell itself, 

 and others by a tendinous 

 prolongation of the foot, which 

 serves as a byssus. The shell 

 is generally irregular 

 form, in consequence f these 

 adhesions ; its hinge is very 

 analogous to that of Unio, the 

 left valve being provided with 

 a tooth, and further back with 

 a projecting plate, received into corresponding cavities in the 

 right valve. The foot is generally small, and the adductor muscle 

 is double, the anterior one, however, being sometimes rudi- 

 mentary. The Chama is attached by the shell itself to rocks, 

 corals, and even to masses of similar shells, in the manner of 

 Oysters; and the individuals are thus cemented so strongly to 



Fir,. 695. CHAMA, with the shell remoTed, 

 to show the arrangement of the mantle and 

 its orifices. The two lobes are adherent along 

 their entire edges ; except at the respiratory 

 passages, r and , and to give exit to the 

 foot,/. 



