1 14 Mr. Barnes on the Genera Unio and Alasmodonta. 



themselves buried in the mud, having their beaks turned 

 downward."* If he means by this that they are usually 

 concealed, or that they he on their beaks ; we remark 

 that, as it regards those of our country, such is not the fact. 

 In winter they may bury themselves, but in summer we 

 have found them, generally, when at rest, standing with 

 the posterior side inserted obliquely, and the hinge margin 

 the anterior slope, and a small portion of the basal edge ex- 

 serted. Even when they sink below the surface the place of 

 their retreat is conspicuous. In streams which have a rough 

 bottom, and rapid current, they choose the narrow crevi- 

 ces between the stones or under the edges of rocks, and 

 thus defend themselves from injury. We have never found 

 a live Unio on its hack, or on what M. Lamarck and his 

 followers would call the hase. 



While standing in the position above described, they have 

 the anterior side slightly gaping, but on being touched they 

 instantly close. They are usually found in company, rare- 

 ly solitarj ; and the sand of the bottom is often marked 

 with little furrows made by their passing from place to 

 place. They advance with the posterior end foremost, and 

 the decorticated beaks, seen through the water, bear a 

 strong resemblance to the eyes of a large animal. Deter- 

 ville says "they have been observed to live for several 

 months of the summer in clay too hard to be cut by the 

 hoe, and with but momentary showers to refresh them." This, 

 if it be a fact, must rest, for the present, on his authority; a-s 

 we know of no one who has confirmed it by observation. 



We know but little concerning the generation and prop- 

 agation of the species of Molluscous animals that inhabit 

 these shells. They are generally supposed to be hermaph- 

 rodite per se. If they are really and absolutely so, the num- 

 ber of species must be exceedingly great. M. Lamarck 

 supposes that they are propagated by means of a fecunda- 

 ting fluid enjitted into the water. If so, they must be male 

 and female. What reason he has for this supposition, we 

 are not informed, but if it be admitted, it will readily ac- 

 count for the numerous varieties of these animals, and it 

 will show also that they are merely varieties, and not dif- 

 ferent species, that is, they will pcove to be the "common 



* Ih se tienrient enfonces dans la vase, ayant leur crochets tonrnes en has.'" 

 ■—Lam An. S. Ferlebres, FoL WL page 70. 



