» PROCEEDINGS OF THE NATIONAL MUSEUM. vol.61. 



is on the western slope of the Cordillera Oriental and is separated by 

 high mountains from the great areas drained by the Orinoco and 

 Amazon Eiver systems. 



Externally this shell closely resembles many of the arched speci- 

 mens of the common Margaritana marcjaAtifera of the Northern 

 Hemisphere. Its relation to Anodontites of the tenehricosus group 

 is quite evident and its nearest ally in that group is A. dessini 

 Fischer. The constriction in this species is much more marked 

 than in any of the other species of the group. The degree of con- 

 striction increases with the age of the shell. One specimen, ap- 

 parently quite old, is much more constricted than the type. 



The character of the periostracum is very peculiar on account of 

 the great number of fine wrinkles. At fii-st glance these wrinkles 

 appear to be due to shrinkage, but the great number of them would 

 call for an amount of shrinkage that would seem to be unreason- 

 able. The occurrence of similar wrinkles, though in obscure form, in 

 the nacre indicates that they arise from the structure of the animal 

 and are not due to shrinkage. All four of the specimens at hand 

 are wrinkled in the same way. 



MONOCONDYLAEA FELIPPONEI. new species. 



Plato 1, figs. 2, 13 ; plate 2, figs. 4, 11 ; plate 3, fig. 7. 



Shell much inflated, subrotund in form, narrowed anteriorly, very 

 wide and nearly- squarely tnmcate posteriorly. Posterior dorsal 

 ridge rounded, not angled, but the ridge thrown into prominence by 

 a radiating line of dark color. Ventral edge broadly rounded, ob- 

 scurely angled at its middle nnd at its posterior end. Anterior end 

 slightly gaping. Posterior margin nearly at right angles with tiie 

 dorsal margin. Periostracum thick, clothlike, without gloss, its sur- 

 face much roughened by niunerous concentric lamellae. Color dark 

 olive green. Interior somewhat iridescent, wdth a more or less rosy 

 tinge throughout, which is more marked in the middle portion. 

 Parts of the interior with a greenish tinge. Antei'ior adductor scar 

 deeply impressed, posterior scar superficial. Hinge line undulating, 

 the single tooth in each valve knoblike. Prismatic border very broad 

 ventrally, very narrow elsewhere. 



The type (Cat. No. 340663, U.S.N.M.) measures: Length 33 mm., 

 height 27 mm., diameter 19 mm. It comes from Barra del Arroyo 

 Sacra, Department of Paysandu, Uruguay, and was collected and 

 presented by Dr. Florentino Felippone. 



This species is quite different from any M onocondylaea hitherto 

 described. It seems to stand intermediate between M. paraguayana 

 Lea and M. franckcana Moricand. 



