TRIBE NAYADES. 173 



CASTALIA. 



EquivaJve, inequilateral, tr^igonal ; the umbones decorticated, and 

 injlexed before ; hinge with two la?nellar transversehf striated 

 teeth, one posterior, remote, abbreviated and subtrilamellar ; 

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



C. QuADRiLATERA. D' Orb. Am. t. 7^. 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. A\. — S. America. 



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

 Braz. t. 2b. 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. I|^..2^. — S. America. 



C. Inflata. D'Orb. Am. t. 72. /. 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 sti-iated. 1 i . . 15.. — 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 sometiynes none whatsoever, or having irYegular granulated 

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

 terior muscular impression ; the umbo usually decorticated. 



UNIO. 



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

 cated, and somewhat worn ; posterior muscular impression 

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

 tooth short, irr'egular, simple or cleft, substriated ; the lateral 

 elongated, compressed aiid j^i'oduced when aged ; ligament 

 external.^ 



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

 filled up in this monograph. 



