TRIBE NAYADES. 173 



CASTALIA. 



JEquivalve, inequilateral, trigonal ; the umbones decorticated, and. 

 infiexed before; hinge with two lamellar transversely striated, 

 teeth, one ])oste?'ior, remote, abbreviated and subtrilamellar ; 

 the other anterior, elongated, and lateral ; ligament external. 



C. Quadrilatera. D' Orb. Am. t. 73. Subquadrilateral, 

 inflated, thick, brown, when young entirely covered with broad flat 

 elevated ribs (which in the adult disappear near the margins) beaks 

 prominent, and posteriorly recurved ; anteriorly with a prominent 

 keel having the appearance of an acute angle : inside brilliant white, 

 only the anterior tooth striated, the cardinal being infinitely 

 divided. 4^. — S. America. 



C. Ambigua. Lam. 1. — Tetraplodon Pectinatum. Spix. 

 JBraz. t. 25. f. 3, 4. Ovate-angular, inflated, trigonal, brown, pos- 

 teriorly short, the beaks not recurved, anteriorly with a rounded keel, 

 at the beaks covered with irregular carinated ribs : shining white, 

 teeth striated. l|-..2-J. — S. America. 



C. Inflata. D'Orb. Am. t. 72. f. 4. to 10. Ovate, very 

 inflated subquadrilateral, very thick, greenish brown, with strong 

 ribs at the umbones and usually transverse striae ; beaks prominent : 

 posteriorly very short and rounded ; anteriorly elongated, smooth, 

 having the appearance of two prominent obtuse angles : inside 

 bluish white, teeth striated. 1 \ . . 14. — S. America, Guiana. 



TRIBE NAIADES. 



Shells inhabiting fresh waters ; the hinge sometimes having an irre- 

 gular, simple, or cleft tooth, and a longitudinal, prolonged one, 

 and sometimes none whatsoever, or having irregular granulated 

 tubercles the whole length of the upper part ; a compound pos- 

 terior muscular impression ; the umbo usually decorticated. 



UNIO. 



Shell transverse, equivalve, inequilateral, free ; umbones decorti- 

 cated, and somewhat worn ; posterior muscular impression 

 compound ; hinge with two teeth in each valve ; the cardinal 

 tooth short, irregular, simple or cleft, substriated ; the lateral 

 elongated, compressed and produced when aged ; ligament 

 external.* 



1 The outline of Mr Lea's Synopsis of the Uniones has been 

 filled up in this monograph. 



