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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 Linnsean bond of 
relationship, the hinge. This genus, indeed, were 
it formed only upon the name, derived from 
a gaping, would exclude a large proportion of its 
shells, and would properly retain only one species, 
C. Grigas, which is the largest of all testaceous 
