NEW EXOTIC UNIONISE. 383 



Hob. — Uruguay River, South America, Prof. J. Wyinan. 



My cabinet. 

 Diam. S, Length 1:4, Breadth 1-9 inch. 



Shell smooth, subrotund, somewhat inflated, inequilateral, rounded before and 

 behind, slightly flattened on the sides; substance of the shell somewhat thick, 

 rather thicker before ; beaks a little prominent, with diverging undulations at the 

 tips ; ligament short, rather thin and dark-brown ; epidermis dark olive-brown, 

 striate, somewhat shining, with distant marks of growth ; umbonial slope inflated 

 and rounded ; posterior slope rather contracted, slightly sulcate, raised into a well- 

 defined carina ; cardinal teeth rather small, compressed, striate and somewhat 

 lamellar ; lateral teeth rather long, somewhat curved and terminating somewhat 

 abruptly ; anterior cicatrices rather large, well impressed, confluent with the lower, 

 but distinct from the upper ; posterior cicatrices rather large, confluent and slightly 

 impressed ; dorsal cicatrices placed across the centre of the cavity of the beaks ; 

 cavity of the shell rather shallow and wide ; cavity of the beaks shallow and 

 rounded ; nacre white and iridescent. 



Remarhs. — This species is of a graceful form and appearance, and probably is never 

 large. A single specimen only was received, which has four distinct growths. It 

 is two inches wide. Its outline is disposed to be subtriangular, but it belongs more 

 to the subrotund group, which so much prevails in South America. In this speci- 

 men the posterior portion of the cardinal teeth in both valves is slightly divided so 

 as to give it at that point the appearance of the small teeth of the genus Tellina. 

 This may not, however, be found to be characteristic and prevail in other indi- 

 viduals. There is a slight disposition in the thickened part of the nacre of this 

 specimen to be yellowish. The beaks are not entirely perfect, but they display 

 diverging undulations for a short distance to exist. The very obscure rays which 

 may be observed by close inspection in this specimen, are accompanied by very 

 obscure raised lines, which pass from the undulations of the beaks to the basal 

 margin. This character is not uncommon in some of the South American subrotund 

 'species, but it does not prevail as a permanent character, I believe, with any of the 

 species. This species is less rotund than any of the rotund group in this paper, and 

 it is higher over the umbonial slope and the portion anterior to it. It is not very 

 far removed from patelloides, herein described, but is not so lenticular as that species. 



Unio patelloides. PI. 43, fig. 291. 



Testa lasvi, subrotunda, subcoiupressa, subfequilatcrali, aDtice et postice rotundata; valvulis subcrassis, 

 autice crassioribus ; natibus prominulis, ad apices divaricate uudulatis ; epiderruide tenebroso- 

 castanca, striata, eradiata ; dentibus cardinalibus longis, comprcssis, obliquis, crenulatis corrugatis- 

 que ; lateralibus longis. crenulatis curvisque ; margarita argentea et iridescente. 



