17 



knowledge of these animals, I subjoin a de- 

 scription of that inhabiting the U. nigra. 



All the animals belonging to this family pre- 

 sent but slight differences in colour, dimensions, 

 and proportions. 



Body white or slightly flesh coloured ; mantle 

 smooth, thin, covering the valves, bilobed and 

 posteriorly notched, rough, without fringes ; se- 

 cond mantle interior, branchial, striated obliquely, 

 thin, bilobed posteriorly, much smaller than the 

 exterior one, and enveloping the foot ; foot com- 

 pressed, muscular, coriaceous, oblong, dilatable ; 

 mouth anterior ; anus posterior, at the extremity 

 of the ligament. The anterior lateral syphons 

 equal, one on each side, behind the mouth, in the 

 form of a perforated tubercle ; and still further 

 back, equally on each side, an obtuse bilamellar 

 appendage with unequal plates, flat, oval, or ob- 

 long, the interior one largest ; these are appa- 

 rently the organs of generation. From this 

 description, the result of an accurate examination 

 of twenty species and three hundred individuals, 

 it will be perceived, that there is a very remark- 

 able difference between these animals and those 

 of the European Unios, as described by authors, 

 and particularly by FERRUSA.C, (vide Essai d'une 

 methode conchyliogique) who prides himself on 

 the scrupulous exactness with which he has de- 

 scribed the fluviatile mollusca. 



