teeth. This I have termed the hinge margin. The form of 

 the notch or sinus which terminates this part will also be 

 fomid of much importance in discriminating the species; 

 for no shells vary more in their form, thickness, or convexity 

 than these do, according to their locality, age, or other 

 circumstances. 



Shell transverse, oval ; rather thick and ventricose ; both 

 extremities obtuse ; the anterior side (from the umbones to 

 the exterior margin) obliquely rounded; umbones promi- 

 nent ; hinge margin rather thick, slightly curved, and swelled 

 immediately under the umbones ; sinus short, abrupt, curved ; 

 epidermis coarse, black, and much wrinkled ; inside stained 

 with yellow, and having a narrow reddish rim or margin. 



For this species, now, I believe, first made known, I am 

 indebted to G, C. Bainbridge, Esq. of Liverpool, who re- 

 ceived several specimens from the United States. It ap- 

 pears to have been unknown to Mr. Say, who has published 

 an account of the land and river shells of North America. 



The student might be led to suppose, that the two genera 

 of U)iiu and Anodon are strongly characterized ; for the first 

 includes many of the most ponderous bivalves yet discovered, 

 and the second some remarkably thin and brittle. Among 

 the Uii/ones are shells furnished with hinges of the greatest 

 force, while most of the xiuudons are perfectly destitute of 

 any ; nevertheless, the gradations by which these characters 

 approach each other are very remarkable, and some shells 

 which partake of both have been arranged in separate 

 genera. Of these, the best defined are Hi/ria Lam. and 

 Dipsas of Leach ; the one allied nearest to Unio, but having 

 the cardinal teeth assuming the appearance of lateral or 

 lamellar teeth ; the other more resembling the Anodons, 

 but furnished with a strongly defined and elevated lamellar 

 tooth, extending the whole length of the hinge. Between 

 these two genera should be placed another of Lamarck's, 

 called by him Iridinia, which has likewise only a simple 

 lamellar plate, but broken into a great number of crenated 

 teeth. The observing Mr. Say has likewise proposed another 

 under the name o^ Alasmodonta, which, however, I shall take 

 another opportunity of noticing. 



I have ventured to exchange the ungrammatical name of 

 Anodonta, given by Bruguiere to this genus, for Anodon, at 

 the suggestion of the learned Dr. Goodall, Provost of Eton 

 College. 



