THE UWBOES OF BIVALVES. 389 



proceeds, is generally a curved, swelled portion on the dorsal 

 margin, and is named the umbo, or beak. All the lines^ 

 ribs, and streaks of colour, which commence and diverge 

 from these points towards the ventral margin, are described 

 2i^ radiating ; all those which are formed in rings round 

 the nucleus, — that is, in the direction of the lines of growth, 

 — are named concentric. 



The anterior of the shell may be known from the posterior 

 by the mnloes turning towards it; or, if there is a ligament 

 visible externally, it is on the posterior side. 



Formerly a different system of measurement and descrip- 

 tion was used ; and, lest it should mislead those who find it 

 ill Lamarckian works, it will be desirable to explain it here. 

 By that system, what is now known to be the posterior was 

 described as anterior, and vice versa. The ventral margin 

 was turned upwards, and the margin with the hinge was 

 named as the base. The length was measured from apex to 

 base, and the width from anterior to posterior. The lines, 

 etc. which we now speak of as radiating, were longitudinal ; 

 and those which we describe as concentric, were transverse. 



THE TJMBOES OE BIVALVES 



Are sometimes central, or placed near the middle of the 



u 



