TRIBE NAYADES. 173 
CASTALIA. 
Equivalve, inequilateral, trigonal ; the umbones decorticated, and 
inflexed before; hinge with two lamellar transversely striated 
teeth, one posterior, remote, abbreviated and subtrilamellar ; 
the other anterior, elongated, and lateral ; ligament external. 
C. QuapritaTeRA. 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. 43.—S. America. 
C. Amaicua. Lam. 1.—TretrarLopon Pecrinatum. Spia. 
Braz. 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. 13..21.—S. America. 
@. Inrrata. “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 strize ; beaks prominent : 
posteriorly very short and rounded; anteriorly elongated, smooth, 
having the appearance of two prominent obtuse angles: inside 
bluish white, teeth striated. 13.. 12.—S. America, Guiana. 
TRIBE NAIADES. 
Shells inhabiting fresh waters ; the hinge sometimes having anirre- 
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.’ 

1The outline of Mr Lea’s Synopsis of the Uniones has been 
filled up in this monograph. 
