191 



2. 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. PL vi. fig. 26. 



4. Iridina. Differs from anodonta-m its hinge line being 

 irregularly tuberculated. Its shell has considerable pearly 

 lustre. 



Chamacece. Inequivalved, irregular, and attached, with 

 one large tooth at the hinge, and two 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. PL 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 ; 



