MOLLUSCOIDEA—BRACHIOPODA. 



173 



bounded on the inside by hinge plates which are united above with 

 the cardinal process. Similar plates, the dental plates, bound the 

 inner side of the teeth ; at times these unite before reaching the 

 bottom of the shell, thus forming a plate or cup upon which are 

 fastened the diductor, adductor and pedicle muscles. This cup is 

 called the spondylium {Gypiduld). In some genera (Metista, etc.) a 

 free plate, of different origin, the shoe-lifter process, occurs in place 

 of the spondylium. The solid or excavated platforms in some of 



the Atremata {Ttimerella, Fig. 

 226) are homologous to the 

 spondylium. A corresponding 

 structure sometimes occurs in 

 the brachial valve where it is 

 known as the cruralium. The 

 muscle areas are more deeply 

 impressed upon the posterior 

 portion of each valve. This 

 area in each valve is frequently 

 divided into halves by a ver- 

 tical plate or septum extending 

 from beneath the beak ante- 

 riorly, never reaching the 

 front of the shell (Fig. 233, s). 

 Curving forward from the 

 posterior portion of the bra- 

 chial valve or from the crura- 

 lium there is a pair of short 

 processes, the crura ; to these 

 in many shells are united more 

 or less complicated calcareous 

 ribbons, the brachidia (Fig. 

 220, 2-6). When spirally coiled as in Spirifer or Zygospira they 

 are called spiralia (Fig. 220-3). 



The valves are opened and closed by muscular action. The 

 pedicle valve is held stationary by means of the pedicle. Through 

 the contraction of two pairs of muscles {diductors or divaricators) 

 extending from the posterior portion of the pedicle valve to the 

 cardinal process on the opposite valve the shell opens, since the 

 cardinal process by its position posterior to the teeth forms a sort 

 of lever with the teeth and sockets as fulcrum, and the front of 



Fig. 220. 1, Dorsal valve of Terebratu- 

 lina septentrionalis with cirrated brachia at- 

 tached; showing relation of calcareous loop 

 which is darkly shaded ; 2, Centronelliform 

 stage of loop of Die las ma turgidum X °» 3> 

 spirals and loop in Zygospira modesta X 2 K 5 

 4, loop and crural plates of adult Dielasma 

 turgidum \ 4 \ $, side view of same X4! 

 6, adult loop of Terebratalia obsoleta. (Af- 

 ter Beecher, and Beecher and Schuchert). 



