MOLLUSCOIDEA — BRACHIOPODS 183 



have begun at the small end or beak of the shell. That this was 

 the initial point of growth is seen likewise by the curving of the 

 growth lines around it. The beak, then, is the oldest portion, 

 while the latest built portion is the edge of the shell away from 

 the beak. 



The pedicle is further inclosed, as it passes out of the valve, 

 by two triangular plates, the deltidial plates, which, therefore, 

 more or less close the delthyrium. A pair of teeth on the pos- 

 terior portion of the pedicle valve fit into corresponding sockets 

 in the brachial valve ; these sockets are also the bases of the 

 calcareous skeleton that supports the arms. Projecting be- 

 tween the teeth of the pedicle valve is a short prolongation, 

 the cardhial process {c.p., Figs. 73 a, 80, 85) of the posterior 

 portion of the brachial valve. 



Since during the growth of the shell the teeth enlarge externally 

 and anteriorly, while probably both resorption and wear take 

 place internally and posteriorly, the hinge line becomes con- 

 tinually broader ; i.e. the teeth move farther apart and, pari 

 passu, calcium carbonate is added to the inside of the sockets, 

 thus causing the hinge line of this valve likewise to become 

 broader. Thus the brachial valve as a whole moves anteriorly. 



Muscles. — The valves are opened and closed by muscular 

 action. The pedicle valve may be said to be stationary be- 

 cause to it is firmly attached the pedicle. Two pairs of muscles, 

 the didiictors, extend from^ the posterior portion of the pedicle 

 valves back to the tip of the cardinal process. Being thus situ- 

 ated, their contraction tends to cause the valves to fly open. 

 The valves close by the contraction of two other muscles, the 

 adductors, which pass transversly from valve to valve. These 

 are elongate and narrow on the pedicle valve, and in passing 

 over to the brachial valve divide, leaving here four more or 

 less circular scars. A pair of muscles extending from the 

 brachial valve and another pair from the pedicle valve with 

 insertion on the pedicle enable the animal as a whole to move 

 in many different directions. 



