32 



INTRODUCTION. 



in the first instance, to describe the position in which it is to 

 be observed, in order to give the student a clearly defined 

 notion of what is intended, when terms expressive of height, 

 depth, length, breadth, &c. are used, as well as when the 

 anterior and posterior extremities are spoken of. For this 

 purpose, we must suppose the animal to be living and creeping 

 along the bed of the sea by means of its foot ; where this foot 

 protrudes, will be the ventral margin, and the opposite part 

 the dorsal margin of the shell. There will then be a valve on 

 each side ; and if we further suppose the animal to be walking 

 forward with its back to the observer, the right and left valves 

 will correspond with his right and left sides. 



Measurements. 



*a£ 7^3 



The length will be measured from anterior (a) to 'poste- 

 rior (jo), and the lines of growth running in the same direction 

 will consequently be longitudinal or concentric ; transverse of 

 some authors. The height will be from the umbones (u), to 

 the ventral margin, and lines or bands in that direction are 

 termed radiating ; longitudinal, according to some authors. 



