.^ 



Mr. Barnes on the Genera Unio and Alasmodonta. 261 



Shell moderately thick; disks flat and compressed, long 

 before and short behind; beaks 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 margin rounded and narrow ; surface 

 deeply wrinkled. Teeth elevated and crenate ; anterior ci- 

 catrix very broad ; posterior composed of three distinct 

 impressions, two small ones before the large one, and also a 

 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. 



Remarks. — The hinge margin is less elevated, and the 

 colour less brilliant, in old than in young and middle aged 

 specimens. The former approach the regular oval form, 

 the latter are broad ovate. 



None of the specimens in our collections exhibit the char- 

 acter mentioned by Mr. Say, viz. "the external laminated 

 tooth obsolete, only one in each valve being perceptible ;" 

 and the tubercles, mentioned by him on the inside, appear 

 only in very old specimens. 



M. Le Sueur thinks that the remarkable union of the 

 valves 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. 



TO XT' ^^ S outline of 



13. UNIO PRJELONGUS. 1 Iff. 11. < ^, 



® ( three sizes. 



Shell much elongated transversely, narrow, thick, 

 tumid, beaks flat ; lateral tooth long, thin ; inside 

 purple. 



