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TRIGONIANA.— Two Genera. 



Trigonia. Shell equivalve, triangular, sometimes suborbicular; cardinal 

 teeth oblong, laterally compressed, diverging transversely, furrowed; two in 

 the right valve, furrowed on each side, and four in the other, furrowed only 

 on one side. Ligament external, marginal. 



T. pectinata, 15 fossil species. 



Castalia. Shell equivalve, inequilateral, triangular; the beaks eroded, 

 recurved posteriorly; hinge with two lamellar teeth, transversely striated; the 

 posterior one distant, shortened, subtrilamellar; the other anterior, elongated, 

 lateral. Ligament external. 



C. ambitjua. 



NAIADA. — Four Genera. 



Fresh icater shells, the hinge of which is sometimes furnished with an irreg- 

 ular, simple or divided cardinal teeth, and a longitudinal one 1 which extends 

 under the corselet, and sometimes no tooth, or is furnished toith irregular, 

 granular tubercles, through its whole length. JMuscular impression posterior, 

 compound ; the beaks decorticated, often eroded. 



Unio. Shell transverse, equivalve, inequilateral, free; beaks decorticate, al- 

 most eroded; muscular impression posterior, compound; hinge with two teeth 

 in each valve; one cardinal, short, irregular, simple, or divided in two, substri- 

 ated; the other elongated, compressed, lateral, prolonged under the corselet. 

 Ligament external. 



U. radiatus, 

 (Gmelin.) 



U. undulatus, 



U. complanatus, 

 (Solander,)hab. 

 rivers and lakes <j rhornbula, 



radiata. Virginiata, Lam. 

 radiatus, Barnes. 

 Say's Am. C. p. ii. f. 8. 



Say's Am. C. p. xvi. 



'purpureus, Say, Am. C. 



pi. iii. f. 1. 

 rarisulcata, Lamarck, 

 coarctata, ' 



purpurascens, ' 



U. ovatus, 

 (Say.) 



ovata, Lamarck, 3 by 4 inches. 

 Say's Am. C. pi. ii. f. 7. 1st ed. 

 Hildreth in Sil. J. v. 14. f. 21. 



E. of Alleghany 

 mountains. 



georgina, ' 



sulcidens, ' 



Caroliniana, Bosc. 

 fluviatilis, Green. 

 i_ carinifera, Lamarck. 



U. cariosus, Huteola, Lamarck. 

 (Say.) Sil- cariosa, ' 



liman's J. < crassus, Say. (old. 

 v. 6. pi. xi. I carinatus, Barnes, 

 f. 10. a. b. (_ ellipticus, ' 



U. nasutus, ^ 



I ^ fir rostrata, Valen. 

 1st ed. J 



(rayed.) 

 (young.) 



