191 



S. Hyria, — An equivalved, obliquely trigonal, eared bi- 

 valve, with a straight and truncated base ; the hinge formed 

 of two spreading teeth ; the hinder, or cardinal, divided into 

 many parts, of which the inner are the smallest ; the other, 

 the anterior or lateral, very long and lamellar ; the ligament 

 external and linear. 



This genus much resembles anodonta in its form. 



3. Anodonta, — An equivalved, inequilateral, transverse 

 bivalve ; the hinge linear, with no tooth ; a smooth cardinal 

 plate, truncated so as to form a sinus at its anterior ex- 

 tremity, forms the base of the shell ; the impressions are 

 two, remote and lateral ; the ligament is external and linear, 

 placed at the anterior extremity in the groove of the hinge 

 plate. Recent. — PI. vi. fig. 26. 



4. Iridina. — Differs from anodonta in its hinge line being 

 irregularly tuberculated. Its shell has considerable pearly 

 lustre. 



CliamacecB, Inequivalved, irregular, and attached, with 

 one large tooth at the hinge, and tw o impressions. 



1. Diceras. — An inequivalved, adherent bivalve, with 

 very large, conic, divaricating beaks, turned in irregular 

 spires ; a large, thick, concave, subauricular tooth in the 

 larger valve ; two muscular impressions. 



One species only has been seen of this shell, and that in 

 a fossil state. 



2. Chama. — An irregular, inequivalved, fixed bivalve, 

 with recurved beaks ; the hinge one thick, oblique, sub- 

 crenulated tooth, received in a pit in the opposite valve ; 

 two distant lateral impressions ; the ligament external, sunk. 

 Recent and fossil. — PI. vii. fig. 17. 



3. Etheria. — An irregular, adherent, unequal-valved bi- 

 valve, with short beaks, as if sunk in the base of the valves ; 

 the hinge without tooth, waved and rather sinuous and 

 unequal ; two distant, lateral, oblong, muscular impressions ; 



