134 MOLLUSCA. LAMELLIBRANCHIATA. 



thickness, whereas the pearly layer may do so throughout 

 the life of the animal. In some genera both layers 

 consist of calcite, in others of aragonite, or one may be of 

 calcite and the other of aragonite. The surface of the 



Fro. 46. Section of prismatic layer of recent Pinna, parallel to the 

 surface of the shell and at right angles to the prisms. Magnified. 



shell may be smooth, or may be ornamented with radi- 

 ating or concentric ribs, with stria? or spines. Often the 

 exterior shows concentric lamellae which are considered to 

 represent periods of growth. The part of the shell at the 

 umbone is that which was first formed and it sometimes 

 shows different characters to the other parts. The 

 margins of the valves may be smooth or crenulated. In 

 many genera the two valves can be completely closed, in 

 others they are always open at some part, and are then 

 said to be gaping; this gape occurs most frequently at 

 the posterior part but sometimes also anteriorly. 



It is of importance to be able to distinguish the right 

 and left valves. In order to do this we must determine 

 first the anterior and posterior margins. When the soft 

 parts of the animal are present this is easily done. But 

 when the shell only is before the observer the points to 

 be noticed are these : 



