AND DESCRIPTIONS OF NEW SPECIES. Ill 



curved, and slightly undulated at the tips ; ligament short and thick ; 

 epidermis brown ; rays obsolete; cardinal tooth large, elevated and 

 widely cleft in the left valve and emerging from a pit in the right 

 valve ; lateral teeth thick and curved in a direction over the cardinal 

 tooth ; anterior and posterior cicatrices both distinct ; dorsal cicatrices 

 situated on the under part of the cardinal tooth ; cavity of the beaks 

 deep and angular ; nacre pearly white and iridescent. 



Remarks. This is rather a rare shell, and being of a group of the 

 species which are known under the general name of Mytiloules 

 (Rafin.), it has been considered merely a variety of that species. 

 Having recently examined this group with very close attention and 

 with the advantage of very many specimens, I am induced to believe 

 that it may with great propriety be divided into four species, viz. my- 

 tiloides (Rafin.), undatus (Barnes), pyramidatus (Nobis), and trigonus. 



Unio Formosus. Plate XVI. fig. 41. 



Testa triangulari, ventricosa, transversa; clivo posteriori subplano; radiis irre- 

 gularibus, interruptis, subacutis; dentibus cardinalibus magnis, lateralibus brevi- 

 bus subrectisque; margaritd alba. 



Shell triangular, ventricose, transverse, nearly flat on the posterior slope; rays 

 irregular, interrupted, and somewhat pointed ; cardinal teeth large ; lateral teeth 

 short and nearly straight; nacre pearly white. 



Hab. Ohio river, T. G. Lea. 



My Cabinet. 



Cabinet of Professor Vanuxem. 



Cabinet of P. H. Nicklin. 



Cabinet of the American Philosophical Society. 



Cabinet of the Academy of Natural Sciences. 



Cabinet of Peale's Museum. 



Diam. 1, Length !•!, Breadth 1-6 inches. 



