CHI&FLY OF THE UNITED STATES. 31 



prominent, undulated at the tips ; epidermis shining, olivaceous, obscurely rayed ; 

 cardinal tooth in the right valve only and very small; nacre whitish and very 

 iridescent. 



Proc. Acad. Nat. Sci., 1863, p. 190. 



^-o&.—Siam, C. M. Wheatley. 



My cabinet and cabinet of Mr. Wheatley. 

 Diam. -5, Length 1-4, Breadth 2-9 inches. 



Shell smooth, transverse, very inequilateral, very much compressed, flattened on 

 the sides, angular behind and rounded before ; substance of the shell very thin ; 

 beaks scarcely prominent, with a few undulations nearly parallel with the dorsal 

 margin ; ligament dark brown, long and thin ; epidermis shining, olivaceous, ob- 

 scurely rayed and with very distinct marks of growth ; umbonial slope very slightly 

 raised, flattish ; posterior slope very much compressed, raised into quite a high 

 carina, making the dorsal line perfectly straight; cardinal tooth compressed, very 

 small and existing only in the right valve ; anterior cicatrices rather large and very 

 slightly impressed ; posterior cicatrices large and scarcely visible ; dorsal cicatrices 

 small and placed behind the cavity of the beaks ; cavity of the shell very shallow 

 and wide ; cavity of the Ijeaks scai'cely perceptible ; nacre whitish and very 

 iridescent. 



Remarks. — This species is very nearly of the same outline with Anodonta exilis, 

 (nobis,) but it has a higher carina and is not quite so transverse. It might be con- 

 founded with that species if it were not the fact that this species has, in the right 

 valve, a small compressed tooth which does not exist in any exilis I have seen, and 

 which of course separates them generically. In the left valve I cannot trace along 

 the dorsal line any elevation or depression, the small comjaressed tooth of the right 

 valve closing over the line of the left. In the specimen before me, the only one I 

 have seen, the nacre is thickened after the second growth, which gives it a clouded 

 appearance, and this is tinted with a slight salmon color. The nacre is so thin and 

 transparent that the cells of the base membrane may be seen over the whole disk 

 with the Stanhope lens, by holding it up to the light. 



The following species from Dr. Kirk, of Edinburgh, are of unusual interest. They 

 are the first which I have seen from Central Africa, and for them I am greatly indebted 

 to his liberality. He accompanied the Zambezi Expedition, under the British Govern- 

 ment, and was chief officer under Dr. Livingstone during five years. There are six 

 in number, all of which I believe to be undescribed. The three Spathce have the 

 peculiar African type, and probably were furnished with syphons. In one of the 

 species we have, for the first time, an alaie type. The three Uniones differ from any 

 type I have heretofore seen from Africa, and they take more of that of India in the 



