218 BRITISH MOLLUSKS. 



erect teeth on the anterior side of the umbo, and elongated 

 marginal teeth, which are sometimes obsolete, on the posterior 

 side. Ligament external. 



Of the Unio or pearl mollusk, we have three species in Britain, 

 two, U. tumidus and pictorum, inhabiting ponds, lakes, and gently 

 flowing rivers, and confined to England, chiefly the southern and 

 eastern parts, and one, U. margaritifer, inhabiting the mountain 

 rapids of all three kingdoms, in places beyond the range of the 

 other two. The posterior teeth in this last species being reduced 

 to mere marginal ridges, it is raised by some authors to the rank of 

 a genus, Alasmodon. All three Uniones produce pearls, but the 

 mountain species, U margaritifer, surrounded by more turbident 

 elements than those that dwell in tranquil waters, is the chief con- 

 tributor, although " one in a hundred might contain a pearl, and 

 about one in a hundred of the pearls might be tolerably clear." 



The animal of Unio merely differs from that of Anodonta in 

 being of a longer subtriangular form, as indicated by the form of 

 the shell ; the foot is perhaps smaller, and the fringed edges of 

 the posterior extremity of the mantle lobes are not quite so irregu- 

 larly jagged. The shell is firmly hinged by strong, erect, interlock- 

 ing teeth on the anterior side of the umboes, and elongated mar- 

 ginal teeth on the posterior, which, as already stated, are in U. mar- 

 garitifer rudimentary ; the want of them is compensated in that 

 species by the presence of a longer and stronger external ligament. 



The general distribution of the British species is peculiar. U. mar- 

 garitifer abounds in the hill countries of Scandinavia, as in Britain, 

 beyond the range of its fellows, whose central area of habitation 

 is in France and Germany. U. tumidus does not range south of 

 the Alps. U. pictorum is diffused throughout Europe, passing into 

 North Africa and Russian Asia. Both species range together in 

 Britain, but are confined to England. In the southern and eastern 

 parts TJ. tumidus and pictorum are comparatively abundant, and 

 they are found as far north as the south of Yorkshire. That they 

 are absent from Scotland is not surprising, but it is difficult to ac- 

 count for their absence from the south and south-east of Ireland. 

 " Taking the features of this distribution into consideration," says 

 Edward Forbes in his own quaint manner, " it seems as if the Unio 

 margaritifer had migrated southwards from some ancient northern 

 centre, whilst the other Uniones and Anodonta advanced westwards 

 and northwards, with unequal pace however, since only the last 

 invaded Ireland." 



