153 



This genus is closely allied to Dipsas, Leach, Hyrixjb and Iridina, 

 Lam., Alasmodonta, nob., and Anodonta, Brug. But the former 

 has lamelliform teeth only ; Hijria Has lamelliform teeth on both 

 sides ; Iridina and Anodonta are destitute of teeth and Alasmo- 

 donta is destitute of lamelliform teeth. To this exposition we may 

 add, as more remotely allied, the Megadesma of Bowd., which has 

 two cardinal teeth in one valve and three in the other, with a re- 

 mote indistinct tooth on each side ; characters that indicate an ap- 

 proach to Cyrena. Some of these genera are so intimately linked 

 together by intermediate species, particularly the Unio and Ano- 

 donta, there being one or more in our streams, that.it is somewhat 

 difficult to determine in which of the two genera it ought to be 

 placed. Consequently, those naturalists who advocate the union of 

 such genera as are connected by intermediate species, will be com- 

 pelled to unite these, and on the same principle they must reject 

 great numbers of genera long since firmly established. 



Unio triangularis, Barnes. — Specific character. Shell tri- 

 angular, very convex ; anterior margin forming a right angle with 

 the disk. 



Unio triangidaris, Barnes. Silliman's Journal, vol. 6, p. 272, 

 pin,fig.ll. 



Unio ctmeatus, Swainson. Tilloch's Magazine, December, 1823. 



Desc. Shell triangular, very convex, gaping at the extremities ; 

 anterior margin veiy much flattened, forming a right angle with 

 the disks ; having longitudinal slightly elevated lines crossing the 

 transverse wrinkles ; ligaments very short ; anterior tip angulated ', 

 posterior margin rounded, destitute of distinct lunule ; epidermis 

 pale yellowish-olivaceous, radiated with green, interrupted into 

 short lines and spots, which are sometimes sagittate, and disappear 

 on the margins of the old shell. 



Obs. It is highly probable that the present species is the cunea- 

 tus of Swainson ; it certainly agrees with his description. But this 

 is of little consequence, as Mr. Barnes' name has the priority, and 

 he has also preoccupied the name which Mr. Swainson has chosen, 

 Mr. Barnes correctly remarks that this shell in shape resembles 

 Alasmodonta marginata, nob. The extraordinary depressions of 

 the anterior margin is very characteristic and readily distinguishes 

 it from other species. As the shell increases in age, this margin 

 becomes a little elevated at tip, as is exhibited in our figures, and 



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