Mr. Barnes on the Genera Unio and Alasmodonia. 261 
Shell moderately thick; disks flat and compressed, long 
before and short behind ; beake depressed ; ligament conceal- 
ed within the valves ; hinge margin very much elevated and 
compressed; basal margin nearly straight; anterior dorsal 
margin incurved or emarginate; anterior margin rounded 
and broad ; posterior sets ie rounded, and narrow ; surface 
deeply wrinkled. T elevated and crenate ; anterior ci- 
catrix very broad ; eatin composed of three distinct 
impressions, two small ones before the large one, and alsoa 
row of very small impressions across the cavity of the beaks, 
before the cardinal tooth. Naker red-purple and very bril- 
liant ; cavity of the beaks small and indented with from six 
to ten minute impressions in a row nearly longitudinal. 
—The hinge margin is less elevated, and the 
colour less ‘brilliany, j in old than in young and middle aged 
specimens. ‘The form - approach the regular oval form, 
the latter are broad ov 
None of the specimens in our collections exhibit the char- 
acter mentioned b ry, viz. “the external laminated 
tooth obsolete, only one in each valve being perceptible ;” 
and the tubercles, mentioned by him on the inside, appear 
ae in very old specimens 
Le Sueur ‘thinks that the remarkable union of the 
08 above the ligament ought to characterize a distinct 
genus. This union can seldom be observed in Cabinet spe- 
cimens, as the part is very fragile. Of the numerous speci- 
mens in our collections, one only retains the full elevation of 
the wing ; the rest having been broken in transportation. 
This is the most splendid species of the Unio yet known, 
and it is so remarkably characterized as readily to be distin- 
guished from all others, except perhaps the Unio Gracilis ; 
which, though perfectly distinct, might, at first view, be mis- 
taken for the young of this species. 
outline of © 
three sizes. 
Shell much elongated transversely, narrow, thick, 
tumid, beaks flat; lateral tooth long, thin ; inside 
purple. 
13. unio pratoneus. Fig. 11. 
