20G AMERICAN JOURNAL 



rather thick; beaks not prominent, eroded, having no indica- 

 tion of any undulations at the tip; epidermis light brown, 

 with occasionally broad green rays on the posterior slope ; the 

 umbones, and, indeed, the whole surface is covered with 

 erowded, fine, crenulose striae, which on the posterior slope 

 are crossed at right angles with prominent, regular folds; car- 

 dinal teeth prominent, erect, bilobed in the left valve; anterior 

 cicatrices deeply impressed, distinct; posterior cicatrices, con- 

 fluent and indistinct; dorsal cicatrices small, but distinct, and 

 placed at the base of the prominent tooth in the left valve, 

 while in the right valve they are less distinct and placed a 

 little more posteriorly; nacre light salmon, iridescent. 



Dimensions. — Length 2'1 inches, breadth 145 inches, diam. 

 •5 inch. 



Habitat. — Pegu. 



Museum Comp. Zoology, Cambridge, Mass. Cabinet of 

 Huo-h Cuming, London. Cabinet of G. "W. Tryon, Jr. Cabi- 

 net "of C. M. Wheatley. 



Observations. — With Monocondyloea Peguensis herein de- 

 scribed, came also several fine specimens of this interesting 

 shell. At first they were supposed to be merely the young of 

 Peguensis, but, on examination, were found to differ in many 

 esssential particulars; they are uniformly more depressed, 

 lio-hter colored, more regularly rhombic in form, and of less 

 size • but the most prominent difference consists in the beauti- 

 ful crenulose striae of the present species, which are densely 

 crowded over the whole surface of the shell, while M. Pegu- 

 ensis is remarkably smooth over all except the posterior slope, 

 and there the rugose folds are broader and less numerous than 

 in M. crebristriata ; the lines of growth are also more nume- 

 rous in M. Peguensis, generally about 8, while crebristriata has 

 rarely more than 3. 



