INTRODUCTION. 33 



The points from which the growth of the shell commences, 

 are called the umbones ; these usually turn towards the anterior 

 part of the shell : if this circumstance fails to point out the 

 anterior, it may in many cases be distinguished by the mus- 

 cular impressions of the mantle. If this has a sinus or winding, 

 it is always near the posterior muscular impression ; and in 

 all cases where there is an external ligament, it is on the pos- 

 terior side. 



There is sometimes an impression near the front of the 

 umbones, which forms a semicircle on each valve ; the space 

 within this semicircle is called the lunule (wood-cut, fig. 71 

 and 72, I. I. I.) ; a corresponding depression, when it exists 

 on the posterior margin near the umbones, is named the 

 escutcheon. 



Hinge. 



Fig. 73, I t, lateral teeth ; c t, cardinal teeth; c, cartilage under 

 the ligament ; I, ligament ; /. fulcrum of the ligament. 



The hinge of the shell is on the dorsal margin, and is com- 

 posed of the various apparatus by which the two valves act 

 upon each other in opening and shutting It consists of a 

 ligament , which is placed on the dorsal margin, just at the 

 back of the umbones, and unites the two valves together ; the 

 cartilage or thick gristly elastic substance, sometimes found 

 close to the ligament, to which it then forms an inner coating, 

 and sometimes received into a pit within the shell. It serves 

 the purpose of keeping the shell open when not forcibly closed 

 by the adductor muscles. An inner layer of shelly matter upon 



D 



