LNIO. 



with a more or less dark- coloured olivaceous epidermis, 

 wliicli is ill almost all cases eroded or eaten away from 

 tlie iimbones, and the more prominent parts of the shells; 

 a character which it possesses in common with all fresh 

 water bivalves. The substance of the shell is highly bril- 

 liant and of a pearly lustre. Hinge tooth i^hort and irre- 

 gular, simple or double, sometimes more or less divided 

 and striated ; in the species which Lamarck calls Castalia 

 ambigua, it is slightly elongated and more regularly stri- 

 ated than in most others, but it is far from being regu- 

 larly subtrilamellar. This hinge tooth which is very irre- 

 gular and thick, in those species whose shell is thick ; 

 more regular and less clumsy in the thinner species, appears 

 to be the only constant character by which Unio is to be 

 distinguished from Anodon; for although, as its name 

 implies, that Genus is wholly destitute of teeth, yet there 

 are some shells which evidently combine all the other 

 characters of both genera : and further, the two anterior, 

 produced or elongated, and compressed lateral laminae 

 or teeth, which usually characterize Unio, are sometimes 

 very indistinct, sometimes entirely obsolete ; and also 

 subject to the variation above described, according to the 

 degree of thickness in the respective species. Muscular 

 impressions two in each valve, the posterior one com- 

 pound. Ligament external, more or less elongated ac- 

 cording to the transverse length of the shell. 



This is a Genus of fresh water shells, which appears 

 to be very numerous, and whose species are exceedingly 

 difficu.lt to determine, because subject to so much varia- 

 tion. Lamarck describes forty-eight species, of which 

 several are British, of these the margaritifera is the most 

 celebrated; it sometimes affords pearls of considerable 

 value even in our rivers : it is difficult to say, whether 

 the pictorum of Lamarck, and the ovalis of Montagu, 

 are the same species ; the latter, and the U. Batavus, are 

 common. Of the species which inhabit the rivers of 

 warm climates, many are remarkable for the beauty of 

 their internal colouring, their pearly lustre, and the great 

 thickness of their shell ; the rarer species are very highly 

 valued. 



There are many fossil shells, particularly in the coal 

 measures, which are referred to this Genus, and we think 

 correctly so, because, though we have never been able to 



