CHIEFLY OF THE UNITED STATES. 273 



EemarTcs. — A single specimen only was received, and that without the soft parts. 

 It differs from all the South American species described by D'Orbigny and Moricand 

 in being of almost a perfect lenticular form. It is about the size of Franciscana, 

 Mori., but cannot be confounded with that species, which is lighter and thinner in 

 the substance of the shell and much more inflated and higher in the carina. The 

 specimen before me has the greatest transverse axis about the middle of the disks, 

 and the enlarged or thickened nacre towards the base is unusually great. The 

 greenish-blue of the tips of the beaks arises from epidermal deposit on the interior, 

 which in this specimen is quite remarkable. The tooth of the right valve shuts in 

 behind that of the left valve, which is usual with South American species. 



MoNOCONDYL(EA Pazii. PL 36, fig. 88. 



Testa Ifevi, obovata, inflata, valde inseqiiilaterali, postice rotundata, antice curta rotundaque ; valvulis 

 crassiusculis, antice aliquauto crassioribus ; natibus prominentibus, tumidis, retusis ; epidermide 

 tenebroso-oliva, striata, eradiata; dentibus cardinalibus subcrassis, compresso-tuberculatis, sub- 

 elevatis ; margarita alba et valde iridescente. 



Shell smooth, obovate, inflated, very inequilateral, rounded behind, short and round 

 before ; valves somewhat thick, slightly thicker before ; beaks prominent, swollen, 

 retuse; epidermis dark olive, striate, without rays; cardinal teeth rather thick, 

 compressed tuberculate and somewliat raised ; nacre pearly-white and iridescent. 



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

 Eah. — South America, Don Patricio M. Paz. 



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

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



Shell smooth, obovate, inflated, slightly flattened on the umbones, very inequi- 

 lateral, rounded behind, round and short before ; substance of the shell somewhat 

 thick, slightly thicker before ; beaks prominent, swollen and drawn backwards ; liga- 

 ment rather small and dark brown ; epidermis dark olive, rather coarsely striate and 

 without rays; umbonial slope rounded and very much inflated ; posterior slope sub- 

 cordate, subcarinate, with two indistinct lines on each valve from the beaks to the 

 posterior margin ; cardinal teeth rather thick, compressed tuberculate and somewhat 

 raised ; anterior cicatrices confluent, rather large and moderately well impressed ; 

 posterior cicatrices confluent, large and very slightly impressed; dorsal cicatrices 

 concealed within the cavity of the beaks ; cavity of the shell very deep and rounded ; 

 cavity of the beaks rather deep and angular ; nacre pearly white and very iridescent. 



Soft parts. — Branchial uterus occupies nearly the whole width of the inner 

 branchise, the ovisacks being closely approximate and filled with very minute round 

 ova, not sufficiently advanced to give any idea of the form of the embryonic shell. 



