72 ZOOLOGY. 



although it is attached (indifferently by either valve), it is provided with alarge 

 foot. Before the mollusc was known, it w\as placed near Chama, but its affinities 

 are with the Unionidge; it resembles Iridina in having the extremity of the 

 branchiae united by a small connexion forming the superior siphon. The shell 

 is nacreous, covered with a periostraca, and formed like some species of Ostrea, 

 with laminated cavities, which are so large that the shell is proportionally 

 lighter than that of any other species. They inhabit the Nile from the first 

 cataract upwards, and the Senegal two hundred leagues from its mouth ; and 

 Dr. Golieen brought several imperfect specimens from the upper St. Paul's 

 in Liberia, several hundred miles from the sea. Several species are known. 



The genus Mulleria is said to be founded upon young specimens of Etheria. 



Fam. 5. Carditidm. In Cardita the mantle is united for a short space 

 posteriorly, forming a short anal siphon. It has three or four pair of trun- 

 cated palpi ; the shell is suborbicular or transverse, generally ribbed, and 

 the two cardinal teeth are oblique. This genus occurs plentifully in a fossil 



state. 



** Cardiacea. 



The sub-section Cardiacea of the section Dimyaria contains the families : 

 1, Chamidne; 2, Tridacnid^ ; 3, Cardiid^e; 4, Cycladidas; 5, Tellinida^ ; 6, 

 Lucinidse ; 7, Yeneridse ; 8, Crassatellidse. In these the mantle is closed 

 posteriorly, and the siphons are united or distinct. 



Fam. 1. Gliamidm. This family includes Chama, Diceras (somewhat 

 doubtful as a distinct genus), and Cleidothosrus. In Chama the shell is 

 irregular, spinous or rough, and attached, with the valves unequal and the 

 umbones involute. The hinge has an oblique stout tooth fitting into an 

 opposite ca^dty. The species present many varieties, chiefly from taking 

 the form of the bodies to which they affix themselves. Cleidotha^rus 

 resembles Chama, but is remarkable for having an internal accessory piece 

 held by a portion of the ligament. 



Fam. 2. Tridacnidw. This family is represented by the genus Tridacna, 

 wdiich includes the largest species of shell known, Tridacna gigas {jyl. 76, 

 jig. 33). The position of the animal in the shell differs from that of most 

 bivalves, as the foot passes through an opening in the lunule upon the closed 

 or hinge margin of the shell ; to effect wdiich the position of the animal is 

 changed so as to bring the open foot of the mantle above. The mantle is 

 nearly closed, leaving a small anal aperture, a large branchial one, and a 

 third corresponding to the lunule, for the foot, which is large and byssiferous. 

 The anterior adductor muscle is obsolete, or confounded with the large 

 posterior one, the two being very close together, forming a single 

 impression. 



The shell of Tridacna gigas is said to attain a length of three or four 

 feet, and a weight of 300 pounds. The French name them henitiers., from 

 the use made of them in the Catholic churches to contain holy water. Tlie 

 significant name of the genus was applied by the ancients to certain 

 oriental oysters, which were so large as to require three hltes in eating them. 

 The mollusc of Hippopus resembles that of Tridacna ; and although the 

 shell has no opening, this part varies much in size among the species of 

 276 



