RUPELLARIA. 437 



Genus SAXIDOMUS, Conrad. 



Shell transversely ovate, sub-equilateral, ventricose, solid, 

 cretaceous, or dirty grey ; beaks rather tumid ; lunule and 

 area none. Hinge thick, narrow, 3-4 dentate in each 

 valve ; teeth unequal, narrow, the two anterior the most 

 prominent, the second bifid ; ligament external, very 

 thick, elongated, semi- cylindrical. Muscular impressions 

 large, ovately rotund, nearly equal ; sinus of pallial im- 

 pression large, elongated, horizontal. 



Ex. S. Nuttalii, Conrad, pi. 109, fig. 3, 3, a. 



In this genus there is no lunule. Eight species have 

 been described, from Western America, India, and Aus- 

 tralia. 



Species of Saxidomus. 



decussatus, Desk. opacus, Sow. 



giganteus, Desk. Petiti, Desk. 



maximus, Anton. purpuratus, Sow. 



Nuttallii, Conr. squalidus, Desh. 



Genus RUPELLARIA, Fleuriau de Bellevue. 



Siphons long, unequal, united as far as their middle; 

 the respiratory siphon fringed at the orifice with a double 

 series of cirrhi, the longer ones pectinated ; the anal siphon 

 with numerous short, cylindrical cirrhi. Foot small, coni- 

 cal, linguiform, and byssiferous. 



Shell transversely ovate or sub-globose, inequilateral, 

 generally gaping posteriorly. Hinge narrow, tri-dentate in 

 one valve, in the other bi-dentate ; teeth small, approximate, 



VOL. n. 3 L 



