CONCIIIFERA OK BIVALVES. 53 



Order II. 



DIMYARIA.* 



This order contains a great number of fa- 

 milies, which maybe grouped into four divisions, 

 arising partly out of the more or less per- 

 fect manner in which the valves close upon 

 each other, partly from the shape of the foot of 

 the inhabitant. In the first division, the valves 

 when shut do not come into contact in all 

 parts of the circumference, but leave a space 

 open at one or both ends ; the foot of the animal, 

 also, is large and thick. In the second the aper- 

 ture left is much less, and the foot is small and 

 compressed. In the third the contact of every 

 part of the valves is more perfect, and the shells 

 can most of them be completely closed, and the 

 foot is flat or lamelliform. The fourth division 

 contains shells of a doubtful nature : the mus- 

 cular impressions are two, but the shells are 

 fixed to rocks, so that they greatly resemble in 

 that respect the order Monomyaria. 



* Called sometimes Bimusculosa ; the Latin form of 

 expressing the sense of Dimyaria. Les Conchiferes dimy- 

 aires, Lamarck. 



E 3 



