62 



CHAM A. 

 (Plate VIII. Fig. 2.) 



A. Gaping. B. Closed. 



Shell thick. Hinge furnished with a gibbous 

 callosity, obliquely inserted in a corresponding 

 channel. This callosity or tooth is either simple 

 or crenate, occasionally double or triple. (Fig. 1.) 

 Some species of this genus are truncate, and 

 gape on the posterior margin ; and though all do 

 not so, they agree in being equivalve, and sub- 

 equilateral, in having recurved beaks, and no pro- 

 minent lips. The form differs much, being sub- 

 globose, reniform, cordate, or rhombic. No genus, 

 perhaps, contains two species so dissimilar in out- 

 ward appearance as C. Cor and C. Gigas ; still 

 they are linked together by the Linnaean bond of 

 relationship, the hinge. This genus, indeed, were 

 it formed only upon the name, derived from xw> 

 a gaping, would exclude a large proportion of its 

 shells, and would properly retain only one species, 

 C. Gigas, which is the largest of all testaceous 



