288 POPULAR BBITISH CONCHOLOGY. 



two valves are spoken of as the shell, and each of the pieces 

 is named a valve. The formation of each valve is the same 

 as that of the one piece of which a univalve consists. From 

 the nucleus the growth is effected by concentricr rings, which, 

 being placed very obliquely and inequilaterally, produce 

 only a single curve instead of a spire, and the whole width 

 of the valve constitutes its aperture. This is the manner in 

 which we should describe it if comparing it to a univalve. 

 But in order to view the bivalve correctly, as in itself, we 

 must suppose it in the position of creeping away before us. 

 The part of the shell at the greatest distance would be the 

 anterior side; that nearest to us, the posterior; the valve 

 on our right hand would be the right valve ; and that on 

 our left, the left valve. The edge on which the hinge is 

 placed is the dorsal margin ; and the opposite edge from 

 which the foot protrudes is the ventral margin. 



MEASUREMENTS. 



In measuring the proportions of the shell, the length 

 will be reckoned from the anterior to the posterior ends ; 

 the hreadth from the thickest part of the right valve to that 

 of the left ; and the height from the dorsal to the vejitral 

 margin. The point in each valve, from which the growth 



