166 BRACHIOPODA 



generally five or six pairs of muscles. In the genus 

 Magellania there are two pairs of divaricators (fig. 69 

 c, c) and one of adductors (a). Both pairs of the former 

 are attached to a process (the cardinal process, fig. 72 B, 

 c, c') on the dorsal valve between the teeth sockets, and 

 one pair join the ventral valve near its centre (fig. 72 A, c), 

 while the other pair, which are smaller, are attached 

 nearer the posterior border (</). Hence the dorsal valve 

 forms with these two pairs of muscles a lever of the first 

 order. The adductor muscles are united to the ventral 

 valve near the centre (fig. 72 A, a) and form a single 

 impression divided by a median line ; these muscles 

 bifurcate before reaching the dorsal valve and there form 

 four impressions (fig. 72 B, a, a'). There are also muscles 

 attached to the peduncle which serve to move the shell 

 bodily, one pair of these being united to the dorsal valve 

 (fig. 72 B, b"), the others to the ventral (A, b, b"). In 

 the Inarticulata the muscles are usually more complicated; 

 thus, in Lingula (fig. 76) we find, in addition to the 

 adductors and divaricators, muscles for moving one valve 

 backward or forward in relation to the other, and others 

 for giving a slight rotary motion. 



The arms, already mentioned as occupying in most 

 genera the main part of the mantle-cavity, are generally 

 coiled up. In some forms they can be protruded a greater 

 or shorter distance. Sometimes they are supported on a 

 calcareous framework — the brachial skeleton — which is 

 attached to the posterior part of the dorsal valve at the 

 sides of the cardinal process. In Rhynchonella (fig. 84 B, c) 

 the brachial skeleton consists of two short curved processes. 

 In Terebratula (fig. 86) there is a ribbon-like band forming 

 a short loop. In String ocephulus (fig. 88) the loop is more 

 extensive and runs parallel to and near the margin of the 



