UNIO. 31 



In this country very little has been done to advance our 

 knowledge of the Unionidas ; and it would be extremely 

 desirable if naturalists who reside in the country would 

 carefully notice and record any instances of different 

 kinds occurring in the same waters, and whether any 

 interi)iediate forms are found in such localities. 



Genus I. U'NIO^ Philippsson. PL I. f. 5, 6. 



Body elongated, rather ventricose : gills nearly straight : 

 labial paljps ovate. 



Shell elongated, soHd : lateral teeth strong : lumde or heart- 

 shaped depression on the anterior side distinct. 



This genus was founded by Philippsson in 1788 in an 

 inaugural Lecture entitled " Dissertatio historico-natu- 

 ralis sistens nova Testaceorum genera ;" but, owing to 

 the circumstance of its having been delivered at a meet- 

 ing of which Retz was the president, the latter has 

 usually had the credit of founding the genus. From 

 this genus it has been since proposed to separate the 

 Pearl-Mussel, under the name of Margaritanay or Alas- 

 modon, on account of the teeth being less developed ; but 

 there does not appear to be sufficient reason, on con- 

 chological grounds, for this separation. The animals, or 

 bodies, of these so-called genera cannot be distinguished 

 from each other. All the species are, according to 

 Moquin-Tandon, oviparous. 



Many species of Unio have been described by Conti- 

 nental writers ; and even Moquin-Tandon, who is by no 

 means addicted to this kind of manufacture, has ad- 

 mitted no less than eleven. Two of these (viz. U. Bat ants 

 and U. rhomboideus [or littoralis]), which are Avidely dif_ 

 fused throughout France, have not yet been detected in 



* A pearl. 



