DESCRIPTION OF FIFTY-TWO SPECIES OF TNIONID^E. 23 



Diam. 1, Length 1-8, Breadth 2-5 inches. 



Shell bialate, folded nearly all over, triangular, compressed, very inequilateral; 

 substance of the shell somewhat thick, thicker before ; beaks scarcely prominent, 

 with radiating folds at the tips ; ligament concealed, long and dark brown ; epi- 

 dermis dark horn color, in the younger light horn color, without rays and with 

 rather distant marks of growth ; umbonial slope very slightly raised ; posterior 

 slope narrow elliptical and raised into a high wing ; cardinal teeth small, sulcate ; 

 lateral teeth long, lamellar and slightly curved; anterior cicatrices confluent, rather 

 small and well impressed; posterior cicatrices confluent, large and slightly im- 

 pressed ; dorsal cicatrices placed in a row across the cavity of the beaks ; cavity 

 of the shell very shallow and wide ; cavity of the beaks very shallow and rounded; 

 nacre white and iridescent. 



Remarks. — I owe to the kindness of Mr. Stevens three specimens, two adults 

 and young one about one-third grown. The outline is very nearly that of Boykin- 

 ianus (nobis) and it is plicate like that shell nearly all over ; but it diff"ers in being 

 a thinner and smaller shell, and in having more and smaller folds. In the young 

 shell before me the folds extend over the whole disk and some of them take a zig- 

 zag form. This is a symphynote species and the high wing in perfect specimens 

 covers the ligament. In the two older specimens this character is lost, but in the 

 young specimen it is evident that the two valves were connate before and behind 

 the beaks. In all the three specimens the cardinal teeth are much divided and 

 therefore disposed to be sulcate. 



I have great pleasure in dedicating this species to Mr. Stevens, to whom I am 

 under obligations for it and other interesting river shells taken by him in Guiana, 

 viz., Unio fluctigei\ An. tortilis, An. ensiformis and Mel. transversa. 



Unio Jeffretsianus. pi. 7, fig. 20. 



Testa crebre et leviter sulcata, regulariter elliptica, compressa, parum insequilaterali ; valvulis crassi- 

 usculis, antice parum crassioribus; natibus prominulis; epidermide tenebroso-fusca, polita, eradi- 

 ata ; dentibus cardinalibus, parvis, lamellatis, obliquis ; lateralibus longis, lamellatis, subrectls et 

 in utroque valvulo simplicibus ; margarita vel alba vel salmonis colore tincta. 



Shell closely and slightly sulcate, regularly oval, compressed, somewhat inequi- 

 lateral ; valves somewhat thick, slightly thicker before ; beaks a little prominent ; 

 epidermis dark brown, polished, without rays ; cardinal teeth small, lamellar and 

 oblique ; lateral teeth long, lamellar, nearly straight and simple in both valves ; 

 nacre white or salmon colored. 



Proc. Acad. Nat. Sci., 1871, p. 188. 



