72 ZOOLOGY. 
although it is attached (indifferently by either valve), it is provided witha large 
foot. Before the molluse was known, it was placed near Chama, but its affinities 
are with the Unionide; it resembles Iridina in having the extremity of the 
branchiz united by asmall connexion forming the superior siphon. The shell 
is nacreous, covered with a periostraca, and formed like some species of Ostrea, 
with [piaianbes) cavities, which are so large that the shell is proportionally 
lighter than that of any ee species. They inhabit the Nile from the first 
cataract upwards, and the Senegal two hundred leagues from its mouth ; and 
Dr. Goheen 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. Carditide. 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, Chamide; 2, Tridacnidze; 3, Cardiidee; 4, Cycladide; 5, Tellinide; 6, 
Lucinide ; 7, Veneridz; 8, Crassatellide. In these the mantle is closed 
posteriorly, and the siphons are united or distinct. 
Fam. 1. Chamide. This family includes Chama, Diceras (somewhat 
doubtful as a distinct genus), and Cleidotherus. 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 cavity. The species present many varieties, chiefly from taking 
the form of the bodies to which they affix themselves. Oleidotherus 
resembles Chama, but is remarkable for having an internal accessory piece 
held by a portion of the ligament. 
Fam. 2. Tridacnide. This family i is represented by the genus Tridaena, 
which includes the largest species of shell known, Zrzdacna gigas (pl. 76, 
jig. 33). The position of the animal in the shell fe 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 which 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 benzters, from 
the use made of them in the Catholic churches to contain holy water. The 
significant name of the genus was applied by the ancients to certain 
oriental oysters, which were so large as to require three bites in eating them. 
The molluse of Hippopus resembles that of Tridacna; and although the 
Shell has no opening, this part varies much in size among the species of 
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