100 



gated, very oblique; the posterior lobe of the tooth in 

 the left valve rudimentary and remote; muscular im- 

 pressions large, the anterior ones slightly impressed; 

 accessory cacatrix remarkably large. 



Cab. Jl. N S., No. 20427. 



OBSERVATIONS. 



The exterior aspect of this shell is remarkably like 

 that of an Anodonta, and the resemblance is main- 

 tained in the highly polished and iridescent interior; 

 but the teeth are remarkably well defined for so thin 

 a shell; middle aged shells have very nearly the out- 

 line of U. Sayanus, but cannot be confounded with 

 that species, on account of their large size, polished 

 and iridescent interior and very different cardinal 

 teeth. It differs from U. tetralasmus, Say, in being 

 portionally shorter and wanting the important cha- 

 racter of a double lateral tooth in each valve. Like 

 that species, the sayanus and camptodon, it has very 

 oblique plicae on the beaks. 



Several specimens of different ages were sent me 

 from New Orleans by Mr. Josiah Hale. 



UNIO PULLUS. 



Plate LIV.— Fig. 2. 



DESCRIPTION. 



Shell elliptical, inflated, particularly about the 

 umbo and umbonial slope; disks slightly contracted 

 towards the middle; ligament margin elevated, not in 



