﻿EXOTIC UNIONIM. 253 



beak to posterior margin ; cardinal teeth somewhat thick, erect, crenulate and double 

 in both valves ; lateral teeth long and somewhat curved ; anterior cicatrices distinct 

 and well impressed; posterior cicatrices confluent; dorsal cicatrices placed on the 

 under side of the plate and the posterior base of the cardinal tooth ; cavity of the shell 

 rather shallow and wide ; cavity of the beaks shallow and obtusely angular ; nacre 

 silver-white and very iridescent. 



Remarks. — A single specimen only before me is from Mr. Cuming. It is a well 

 characterised species and nearest to aratus (nobis.) It is, however, a larger and 

 stouter species and its axis major is greater in proportion. Like that shell it is 

 covered with sulcations. Its color is of a deeper yellow. It has some characters like 

 melleus, herein described, but it is not so wide, is not so oblique and the sulcations are 

 not so fine as in that species. In the teeth they also differ much. The nacre of 

 Dysonii is remarkable for its beautiful pure whiteness. The anterior section of the 

 cardinal tooth in both valves is somewhat bifid in this specimen, but this may not be 

 the case in all other specimens, as it is rarely found so in other species. It may not 

 be a permanent character. 



Unio Demeraraensis. PI. 39, fig. 133. 



Testa sulcata, rhomboido-oblonga, subinflata, inaequilaterali, postice obtuse angulata ; valvulis crassiusculis ; 

 natibus prominulis, ad apices uudulatis divergentibus ; epidermide olivo-fusca, eradiata ; dentibus 

 cardinalibus crassiusculis, erectis, striatis, in utroque valvulo duplicibus; lateralibus longis sub- 

 curvisque; margarita albS. et iridescente. 



Shell sulcate, rhomboido-oblong, somewhat inflated, inequilateral, obtusely angular 

 behind ; valves somewhat thick ; beaks a little prominent, with diverging undulations 

 at the tips ; epidermis olive brown, without rays ; cardinal teeth not very thick, erect, 

 striate and double in both valves ; lateral teeth long and somewhat curved ; nacre 

 white and iridescent. 



Unio Demeraraensis, Lea. Proc. Acad. Nat. Sci., 1859, p. 152. 



Hah. — Demerara. H. Cuming. 



Cabinet of Mr. Cuming. 

 Diam. -5, Length -9, Breadth 1*5 inch. 



Shell sulcate, rhomboido-oblong, somewhat inflated, inequilateral, obtusely angular 

 behind and rounded before ; substance of the shell somewhat thick ; beaks a little 

 prominent, with small divergent undulations at the tips ; ligament rather long, thin 

 and light brown ; epidermis olive brown, without rays, with very indistinct marks of 

 growth; umbonial slope raised and angular; cardinal teeth not very thick, com- 

 pressed, erect, striate and double in both valves; lateral teeth long and somewhat 

 curved ; anterior cicatrices confluent as regards the lower one, but distinct from the 

 upper one ; posterior cicatrices confluent ; dorsal circatrices placed across the centre 



