153 
This genus is closely allied to Dlpsas, Leach, Hyria and Iridina^ 
Lam., Alasmodonta, nob., and Anodonta, Brug. But the former 
has lamelliform teeth only ; Syria 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 Alegadesma 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 trianqidaris, Barnes. Silliman’s Journal, vol. 6, p. 272, 
Unio cuneatus, Swainson. Tilloch’s Magazine, Decemher, 1823. 
Desc. Shell triangular, very convex, gaping at the extremities ; 
anterior margin very 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. 
Ohs. 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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