AND DESCRIPTIONS OF NEW SPECIES. 107 



culiarity in the habits of the Unio oriens. This shell is possessed 

 of so small a portion of nacre, that in some specimens the epidermis 

 may be said to be as thick as the nacre itself. It is obvious 

 therefore that the rolling of stones and sand carried by the rapidity of 

 the current of the Ohio upon them, would destroy them if they took 

 the same position with other species embedded merely in the surface 

 of the sand. This they avoid, and, burying themselves from six to 

 twelve inches in the sand, can only be discovered by a small round hole 

 at the surface through which they receive their supply of water. 



In the description of U. varicosus*, I ought to have mentioned 

 that I did not hesitate to make use of that name, although already used 

 by Lamarck, having no doubt but that his species was the Masmo- 

 donta undulata\ (Say). 



When making some observations on the family iVazarfes, Vol. III. p. 

 442, I mentioned in a note upon the genus Castalia, that it must be 

 considered as a species of the genus Unio. Having recently procured 

 from Paris a perfect specimen of it, I have given it a close examina- 

 tion, and do not now feel by any means certain that it ought not, in 

 the present received division of the family, to be considered a distinct 

 genus. The crenulations of the cardinal and lateral teeth in this speci- 

 men are very distinct, which was not the case in the single valve which 

 I formerly examined. In this character it has a slight approach to 

 the family Jlrcacea; and Lamarck very justly says, "comme elle semble 

 fluviatilef, elle indique que les trigonees forment une transition des 

 arcacees aux nayades." 



Lamarck, in his description of the Castalia, makes no mention of the 

 position or existence of the muscular impressions of this genus. In 

 examining this character, I have discovered that the same observa- 

 tions made at page 67, in relation to the cicatrix of the extensor mus- 

 cles of the Ilyria avicularis, will equally well apply to the genus Cas- 

 talia, and it is very remarkable that it should be so differently situated 

 from the same cicatrix in the genus Uiiio. 



* See Vol. IV. p. 90. f See Vol. III. page 424. 



I There cannot be a doubt of its being fluviatile. 



