INTRODUCTION. 33 



The points from which the growth of the shell commences, 

 are called the umhones ; these usually turn towards the anterior 

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

 anterior, the opposite may in many cases be distinguished by 

 the muscular impressions of the mantle, the sinus or winding 

 of which, if any, is always near the posterior muscular impres- 

 sion ; and in all cases where there is an external ligament, it is 

 on the posterior side. 



There is sometimes an external 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) ; a corresponding depression, when it exists on the 

 posterior margin near the umbones, is named the escutcheon 

 or posterior area. 



Hinge. 



Fig. 73, 1 1, lateral teeth ; c t, cardinal teeth ; c, cartilage under 

 the ligament ; I, ligament ; f. 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 ; and 

 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 



