106 CHAMBERED. 



- coiled round on tlae back of the valves ; hinge with a thick, 

 crenated, lengthened tooth, in one valve, entering a correspond- 

 ing cavity in the hinge margin of the other; muscular impressions, 

 two in each valve, distinct, lateral. — Obs. The Linnsean genus 

 Chama, included the beautiful sheUs now called Tridacna. These 

 are exceedingly different from the true Chamge, being regular 

 and unattached. The Chama (Tridacna) gigas, when at its full 

 age and development, is the largest shell known. Specimens 

 have occurred weighing upwards of 500 lbs., and measuring two 

 feet across. Diceras may be known from Chama by the spiral 

 horns into which the umbones are produced ; Isocardia, by the 

 regularity of the shells, and it is hardly necessary to mention 

 Spondylus, which may be known by the triangular disc between 

 the umbones ; CleidothsDrus, Stutch, which resembles Chama in 

 general form, has a separate bony appendage attached to the 

 hinge, and may, moreover, be distinguished by its elongated 

 muscular impression. 55 species are contained in Mr. Eeeve's 

 Monograph. E. and "W. Indies. Ch. Lazarus, PI. ix. fig. 153, 



CHAMACEA. Bl. The seventh family of the order Lamellibran- 

 chiata, Bl. containing the genera Chama, Diceras, Etheria, Tri- 

 dacna, Isocardia and Trigonia. 



CHAMACEA, Lam. A family belonging to the order Conchifera 

 Dimyaria, Lam. described as inequivalve, attached, irregular; 

 with or without a single rough tooth on the hinge ; with two 

 lateral muscular impressions in each valve. This family contains 

 the genera — 



1, Chama, Leafy ; umbones spiral. Eig. 153, 



2, Etheeia. Very irregular, pearly, without teeth. Eig. 155, 



3, Diceras. Like Chama, but with the umbones free, produced, 



Eig. 154. 

 CHAMBEEED, When the cavity of a shell is not continuous, but 

 is divided by shelly diaphragms or septa, it is said to be cham- 

 bered. This is the case with the shells of the Polythalamous 

 Cephalopoda, as in the Nautilus (see Introduction) . The character 

 is not confined to these, as it occurs in some species of Spon- 

 dyli, and in several turrited univalves. 



