﻿246 DESCRIPTIONS OF 



it has folds which that species has not. It is evidently a small species. The beaks of 

 the individual before me are very much eroded, therefore the character of that important 

 part cannot be determined, but I have no doubt that perfect specimens will display 

 beautifully undulated tips, as the delicate folds which exist on the posterior slope and 

 which also can be observed on the anterior slope, indicate that the whole of the 

 superior portion is covered with them. 



Unio Thwaitesii. PL 37, fig. 125. ' 



Testa lasvi, regulariter elliptica, subinflata, insequilaterali; valvulis crassiusculis ; natibus prominulis; 

 epidermide castanea, polita, transversa fasciatS, ; dentibus eardinalibus valde compressis, lamellatis, 

 valde obliquis, in utroque valvulo duplicibus ; lateralibus longis, lamellatis curvisque; margarita, 

 salmonis colore tincta et iridescente. 



Shell smooth, regularly elliptical, somewhat inflated, inequilateral ; valves some- 

 what thick; beaks a little prominent; epidermis chestnut brown, polished and 

 transversely banded; cardinal teeth very much compressed; lamellar teeth very 

 oblique and double in both valves ; lateral teeth long, lamellar and curved ; nacre 

 salmon colored and iridescent. 



Unio Thwaitesii, Lea. Proc. Acad. Nat. Sci., 1859, p. 152. 



Bob. — Ceylon. Mr. Thwaites. 



Cabinet of H. Cuming. 

 Diam. -9, Length 1'5, Breadth 2-6 inches. 



Shell smooth, regularly elliptical, somewhat inflated, inequilateral ; subtance of the 

 shell somewhat thick ; beaks a little prominent ; ligament rather long, thin and light 

 brown ; epidermis chestnut brown, very smooth and polished, with somewhat distant 

 marks of growth and with several broad, transverse, yellowish bands, very obscurely 

 rayed ; umbonial slope raised and subangular ; cardinal teeth very much compressed, 

 lamellar, very oblique, slightly striate and double in both valves ; lateral teeth long, 

 lamellar and curved, the inferior division in the left valve being much the larger ; 

 anterior cicatrices distinct, but not deeply impressed ; posterior cicatrices confluent ; 

 dorsal cicatrices placed in the centre of the cavity of the beaks ; pallial cicatrices not 

 well impressed ; cavity of the shell rather deep ; cavity of the beak shallow and 

 rounded ; nacre salmon colored and iridescent. 



Remarks. — A single specimen only was submitted to me by Mr. Cuming, who 

 received it from Mr. Thwaites of Ceylon. It is very remarkable that this shell 

 has so strong a resemblance on the exterior to U. wnbrosus (nobis), from Mexico, that 

 no naturalist would think of separating them if it were not for the difference of the 

 interior characters, and particularly in the teeth, especially the cardinal tooth, which 

 is of a totally different type. In the umbrosus it is massive, pyramidal and strongly 

 crenulate, while in the Thwaitesii it is thin, long, lamellar and scarcely striate. The 



