24 



Unio rostrata, dredged in the river Trent near 

 Beeston, and in the lake at Highfield House. 



Unio Deshaysii, procured in the river Trent near 

 Beeston. 



Unio. — Dubious, 



The following description is from two specimens 

 only, dredged from the river Trent, near Clifton Deeps. 

 Should others be found, there will be little doubt but 

 that it will prove a difl'erent species. However, as I 

 describe them from two solitary examples, and as the 

 Unios are so very various in form, it is requisite to be 

 cautious ere I pronounce them a distinct species. 

 Certainly they are very curious, specimens, partaking a 

 mid-way character between Unio pictorum and Unio 

 margaritiferus. The form is that of an elongated oval. 

 The ligament is narrow, long, and rises. The beaks 

 are eroded, as in Unio margaritiferus, displaying the 

 olive-coloured nacre of the umbones, excepting round 

 the edges, where it is pearly-white. The umbones do 

 not rise above the shell. There are twin teeth on the 

 left valve, very solid, and erect, and a broad erect tooth 

 on the right valve ; these teeth bear a very striking re- 

 semblance to those of Unio margaritiferus. Besides 

 these there are lateral teeth resembling Unio pictorum, 

 but not so much elevated, which Unio margaritiferus 

 does not possess. The colour of the shell is of a 

 greenish-brown, with very faint, almost imperceptibly 

 faint, transverse rays, which converge towards the um- 

 bones. The interior of the valves is a bronze colour 

 very similar to that of Unio margaritiferus. The 

 length of the shell is two inches, the breadth one inch 



