138 UNIONISE. 



The last is generally distributed through England, Scotland 

 and Ireland ; one of the Uniones, that representing the 

 section Margaritana or Alasmodon is partially distributed 

 in the three countries, confining itself to the mountainous 

 portions, and the rapid streams which flow from them. 

 The remaining two are confined to England, and abundant 

 mainly in the south and east. On the continent the 

 Alasmodon abounds in the Scandinavian rivers beyond the 

 range of its fellows, which, however, have an almost uni- 

 versal distribution through central and southern Europe. 

 The former species does not range south of the Alps. 

 Taking the features of this distribution into consideration, 

 it seems as if the Unio margaritifera had migrated south- 

 wards from some ancient northern centre, whilst the other 

 Uniones and the Anodonta advanced westwards and north- 

 wards, with unequal pace, however, since only the last in- 

 vaded Ireland. The fossil contents of the fresh- water 

 tertiaries contemporaneous with or immediately preceding 

 the period of the drift, when the greater part of the British 

 isles was under sea, corroborate this view, for in them 

 we find associated together the Unio tumidus and pictorum 

 and the Anodonta cygnea. 



UNIO. Retzius. 



Shell equivalve, often thick, inequilateral, very variable 

 in shape (oblong in the British species). smooth, or furrowed, 

 or nodulous on the surface, and covered with a coloured 

 epidermis ; beaks eroded. Hinge with primary teeth, and 

 with or without elongated laterals ; ligament external, 

 more or less elongated. Muscular impressions conspicuous, 

 pallial scar simple. 



Animal shaped like the shell, thick, mantle freely 



