DIMYAIRA. 121 



ed terminations; a greenish-brown epidermis, and finely stri- 

 ated ; inside pearly, and of a rich reddish golden-yellow. Three 

 inches broad. Inhabits the rivers of America. 



Genus 7. — VNIO.— Lamarck. 



Generic Character. — Shell transverse, equivalve, inequilateral, 

 free ; umbones decorticated, and somewhat worn ; posterior 

 muscular impression compound ; hinge with two teeth in each 

 valve ; the cardinal tooth short, irregular, cleft, and striated ; 

 tlie other oblong, laterally compressed and produced when aged ; 

 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 rhomboid and attenuated ; 

 the opposite side obtusely acute ; the umbones, somewhat 

 warted ; with a dusky-green epidermis, and concentrically 

 wrinkled. Nearly three inches broad. Inhabits the rivers of 

 Europe. 



TRIBE II. TRIGONACEA. 



With a primary lamelliform, and transversely striated tooth. 

 Inhabits the Ocean. 



Genus 8 CAST ALl A.—Lamarck. 



Generic Character. — Equivalve, inequilateral, trigonal ; the 

 umbones decorticated, and inflexed before ; hinge with two 

 lamillary teeth, transversely striated, one posterior, remote, ab- 

 breviated and subtrilamillar tooth ; another elongated and lat- 

 eral ; ligament external. 



This genus seem to be inhabitants of fresh waters. The substance of the 

 shell is nacreous : the epidermis is strong, and the apices corroded. Most of 

 the species produce pearls. 



Castalia ambigua. — The Ambiguous Castalia. Plate 

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

 tudinally ribbed, with distant transverse strice ; epidermis pale 

 chestnut brown ; inpide pearly. Habitat unknown. 



L 



