CHIEFLY OF THE UNITED STATES. 271 



form the embryonic shell would take. Tha palpi or mouth-lips have that peculiar 

 round form which I have elsewhere described, and which seems peculiar to South 

 American species.* It is to be regretted that this species, with so peculiar an outline, 

 should not have had the embryonic form to examine, as it would probably present a 

 very distinct form. In the specimen before me the posterior basal margin is slightly 

 arcuate, but this character may not strictly pertain to the species, as the lines of 

 growth above do not present this inward curve. The beaks are very much eroded, so 

 much so as to obliterate the usual diverging folds which they no doubt once possessed. 

 The perpendicular truncation of the anterior margin and the beak-like posterior mar- 

 gin are very characteristic of the species. The cardinal teeth are also quite peculiar in 

 their form, being sub-divided into about a dozen sections (denticles) diverging from a 

 point immediately under the tip of the beaks. In outline it is allied to that rare 

 South American species which I described under the name oi paralleloplpedon, from 

 Rio Parana and Rio Plata; but it is a larger species, not quite so transverse, and 

 differs in the color of the epidermis as well as the form of the cardinal teeth. 



Unio Paraguatensis. pi. 35, fig. 85. 



Testa Isevi, elliptica, inflata, sublenticulari, valde infeq[uilaterali, postlce et antice rotundata ; valvulis 

 subcrassis, antice crassioribus ; natibus vix prominentibus ; epidermide viridi-fusca, obsolete 

 radiata ; dentibus cardiualibus crassiusculis, obliquis, compressis, iu utroque valvulo duplicibus ; 

 lateralibus sublongis, lamellatis curvisque ; margarita argentea et valde iridescente. 



Shell smooth, elliptical, inflated, sublenticular, very inequilateral, rounded before 

 and behind ; valves somewhat thick, thicker before ; beaks scarcely prominent • 

 epidermis greenish-brown, obscurely rayed; cardinal teeth a little thick, oblique, 

 compressed, double in both valves ; lateral teeth rather long, lamellar and curved; 

 nacre silvery-white and very iridescent. 

 Proc. Acad. Nat. Sci. 1866, p. 34. 



Huh. — Paraguay, South America, Don Patricio M. Paz. 



My cabinet and cabinet of Don P. M. Paz. 



Difim- 1-2, Length 1-7, Breadth 2-5 inches. 



Shell smooth, regularly elliptical, inflated, somewhat lenticular, very inequilateral, 

 rounded before and behind; substance of the shell somewhat thick, thicker before; 

 beaks inflated, scarcely prominent ; ligament rather small and dark brown - 

 epidermis greenish-brown, obscurely rayed ; umbonial slope rounded and inflated ; 

 posterior slope elliptical, somewhat carinate, with two slightly impressed lines in each 

 valve; cardinal teeth somewhat thick, oblique, compressed, double in the right and 

 disposed to be treble in the left valve; lateral teeth long, lamellar and slightly 



* Trans. Acad. Nat. Rci., vol. 5, p. noi. Observation,?, vol, 10, p, 27. 



