CHIEFLY OF THE UNITED STATES. 65 



which I have examiued are slightly inequivalve, having the right valve very slightly 

 over-wrapping the left about the region of the bealis. This is caused by a slight 

 thickening and elevation on the edge of the left valve under the point of the beaks, 

 which meets with a corresponding depression in the right. The same condition is 

 perfectly observable in the Spatha ruhens, which are perfect. 



MONOCONDTL^A MOUHOTIAKA. PI. 21, fig. 62. 



Testa Isevi, ovata, compressa, valde insequilaterali, antiee rotunda, postice subbiangulata ; valvulis tenui- 

 bus ; natibus prominulis ; epidermide luteo-fusca, eradiata ; dentibus cardinalibus parvissimis, lobatis ; 

 margarita albida et valdc iridescente. 



Shell smooth, ovate, compressed, very inequilateral, rounded before, subbiangular 

 behind ; valves thin ; beaks a little prominent ; epidermis yellowish brown, without 

 rays ; cardinal teeth very small, lobed ; nacre whitish and very iridescent. 

 Proc. Acad. Nat. Sci., 1863, p. 190. 



IBah. — Laos Mountain, Cambodia, Siam, Monsieur Mouhot, per H. Cuming, Esq. 



My cabinet and cabinet of Mr. Cuming. 

 Diam. -7, Length 1-3, Breadth 2-5 inches. 



Shell smooth, ovate, compressed, very inequilateral, rounded before, subbiangular 

 behind ; substance of the shell thin ; beaks slightly prominent ; ligament long, thin 

 and brown ; epidermis yellowish brown, with three or four distant marks of growth ; 

 umbonial slope very slightly raised and rounded ; posterior slope very narrow, raised 

 into a carina ; cardinal teeth very small, lobed ; anterior cicatrices distinct, rather 

 large and slightly impressed ; posterior cicatrices confluent, large and scarcely per- 

 ceptible ; dorsal cicatrices placed in the centre of the cavity of the beaks ; cavity of 

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

 nacre whitish and very iridescent. 



Remarhs. — Two specimens of this new species were sent to me, very kindly, by Mr. 

 Cuming. They differ in the smaller being higher in the carina, almost forming a 

 wing, and in the posterior slope being without color while the larger is there nearly 

 covered with green. Both specimens have small obscure undulations on the umbo- 

 nial slope. Specimens may be found with this character more developed. In outline 

 it is very near to MmocondylcBa Saulcyi (Unio) Bourg. It is also near to M. BoneUii 

 (Unio) Fer. It differs from the latter in being less transverse, in having a higher 

 carina and in having the beaks more medial. I dedicate this species to Monsieur 

 Mouhot, a very indefatigable collector, who fell a sacrifice to his ardor in collecting 

 eight hundred miles from the coast. Among five or six species of JJnionidce procured 

 by him two were new, and it is but justice to him that one should bear his name. 



17 



