CHIEFLY OF THE UNITED STATES. 63 



Unio Laosensis. PL 21, fio. 61. 



Testa Isevi, arcuata, in medio compressa, valde inajquilaterali, antice at postice rotundata ; valvulis sub- 

 erassis; natibus prominulis, suboompressis; epidermide tenebroso-fusca vel rufo-fusca, postice obso- 

 lete radiata ; dentibus cardinalibus parvis, striatis. lobatis ; lateralibiis longis, corrugatis subrectisque ; 

 margarita alba et iridescente. 



Shell smooth, arcuate, comijressed in the middle, very inequilateral, rounded be- 

 fore and behind ; valves somewhat thick ; beaks a little prominent and somewhat 

 compressed ; epidermis dark brown or reddish brown, obscurely rayed behind ; 

 cardinal teeth small, striate and somewhat lobed; lateral teeth long, somewhat 

 rough and nearly straight ; nacre white and iridescent. 

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



Rcib — Laos Mountains, Cambodia, Siam, Mons. Mouhot, per H. Cuming, Esq. 



My cabinet and cabinets of Mr. Cuming and Mr. Wheatley. 

 Diam. -9, Length 1-2, Breadth 3 inches. 



Shell smooth, arcuate, compressed in the middle, very inequilateral, rounded be- 

 fore and behind, disposed to be sulcate before ; substance of the shell somewhat thick ; 

 beaks a little prominent and somewhat compressed ; ligament long and light brown ; 

 epidermis dark brown or reddish brown, obscurely rayed behind and with distant 

 marks of growth ; umbonial slope slightly raised and rounded ; posterior slope long 

 and narrow, slightly carinate, and with a single dark ray from the beaks to the pos- 

 terior margin on each valve ; cardinal teeth small, striate and somewhat lobed ; 

 lateral teeth long, somewhat rough and nearly straight ; anterior cicatrices distinct, 

 rather large and well impressed ; posterior cicatrices distinct, large and slightly im- 

 pressed ; dorsal cicatrices placed above the centre of the cavity of the beaks ; ventral 

 cicatrices small and dotted over the middle of the disk ; cavity of the shell wide and 

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

 iridescent. 



BemarTis. — This new Unio was sent with two other species for my inspection by 

 Mr. Cuming. These were TJ. Myersianus, (nobis,) and U. Samatrensis, (nobis.) 

 There were two specimens only of this new species, which is interesting from its 

 form, and which differs much from any I have seen from Asia. Lideed it very 

 closely resembles in outline two Unionidce from Europe, viz. : Unio cmssits, Retz., 

 and Margaritana margaritifera, (Shum.,) the latter being also an inhabitant of the 

 Northern United States and Oregon. It is a thinner and wider shell than crassus, 

 with much less thick teeth, but has an epidermis very much like it. The outline is 

 very close to some varieties oi margaritifera, and so is the cardinal tooth, and like it 

 it has dotted ventral cicatrices. In both the specimens before me the nacre under 

 the cardinal teeth is irregularly deposited, and in tlie cavity of the beaks it is dis- 



