BRACHIOPODA. 



Ill 



the surface such as striation, or by slight depressions or 

 elevations, these markings are termed the muscular im- 

 pressions. In the articulate brachiopods there are 

 generally five or six pairs of muscles. In the genus Wald- 

 heimia there are two pairs of divaricators (fig. 29 A, c, c') 

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

 attached to a process (the cardinal process, fig. 32 B, c, c) 



FIG. 32. Waldheimia flavescem, Becent. A, Interior of ventral valve. 

 /, foramen; d, deltidium; t, teeth; a, impressions of adductor 

 muscles ; c, c', impressions of divaricator muscles ; b, b", muscles 

 of the peduncle. B, Interior of dorsal valve, c, c', cardinal process ; 

 b", hinge-plate ; s, dental sockets ; I, loop ; a, a', adductor 

 impressions; c, point of attachment of the smaller divaricator. 

 (After Davidson.) 



on the dorsal valve between the teeth sockets, and one 

 pair of these joins the ventral valve near its centre 

 (fig. 32 A, c), and the other, which is smaller, is attached 

 nearer the posterior border (c'). Hence these two pairs 

 of muscles form with the shell a lever of the first order. 

 The adductor muscles are united to the ventral valve near 

 the centre (fig. 32 A, a) and form a single impression 



