138 DIMYARIA. 



Genus 7 — UNIO, — Bruguiere. 



Generic Character Shell transverse, equivalve, inequi- 

 lateral, free ; umbones decorticated, and somewhat eroded ; 

 posterior muscular impression compound ; hinge with two 

 teeth in each valve ; the cardinal tooth short, irregular, cleft, 

 and striated; the other oblong, laterally compressed and pro- 

 duced ; ligament external. 



Section * With a short, thick, primary tooth. ** Primary tooth 

 short, compressed, and frequently crested. 



Unio pictorum. — The Painter's Unio. Plate XVI. fig. 

 3. Oblong-ovate, strong, anterior side rhomboidal and 

 attenuated; the umbones, somewhat warty; with a dusky- 

 green epidermis, and concentrically wrinkled. Nearly three 

 inches broad. Inhabits the rivers of Europe. 



TRIBE II. TRIGONACEA. 



With lamelliform, and transversely striated cardinal teeth. 

 Inhabits the ocean. 



Genus 8.— CASTALIA.— Za/«arcA. 



Generic Character Equivalve, inequilateral, trigonal; 



umbones decorticated, and inflexed behind ; hinge with two 

 lamellar teeth, one of them transversely striated, remote, 

 abbreviated, and subtrilamellar tooth ; the other elongated 

 and lateral ; ligament external. 



The sheUs of this genus seem to be inhabitants of fresh waters. The 

 substance of the shell is nacreous : the epidermis is strong, and the 

 apices corroded. 



Castalia ambigua The Ambiguous Castalia. Plate 



X VI. fig. 4. Ovate, oblique, the umbones truncated ; longi- 

 tudinally ribbed, with distant transverse striae; epidermis 

 pale chestnut brown ; inside pearly. Habitat unknown. 



