BRACHIOPODA 



165 



ventral lobe of the mantle and consists of a pair of plates 

 which, in some cases, coalesce. The pseudo-deltidium is 

 developed at an earlier stage in the life of the in- 

 dividual than the deltidium, and grows from the apex of 

 the delthyrium downwards, becoming fused to the ventral 

 valve. 



The two valves of the brachiopod can be opened and 

 closed by means of muscles (fig. 69); those which open them 

 are called the divaricators (c, c), those which close them, 

 the adductors (a). When the soft parts of the animal have 



Fig. 72. Magellania [ = Waldheimia]flavescens, Recent. 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.) x 1^. 



been removed the places where the muscles were attached 

 to the interior of the shell are indicated by a difference in 

 the surface such as striation, or by slight depressions or 

 elevations ; these markings are termed the muscular 

 impressions. In the articulate brachiopods there are 



