110 BRACHIOPODA. 



generally found on the ventral valve, but may also occur 

 on the dorsal. The margin of the foramen is sometimes 





FIG. 31. Cyrtia exporrecta, Wenlock Limestone, ale, area; b c, 

 hinge-line. Natural size. 



formed partly or entirely by the deltidium (fig. 32 A, d). 

 This consists usually of two small plates ; in Rhynchwiella 

 the two plates form a triangle, in the centre of which is 

 the foramen ; in Waldheimia the two plates form the 

 lower margin only of the foramen ; in Terebratella the 

 two plates are completely separated by the foramen. 



Almost all living brachiopods are fixed to a foreign 

 object, but some of the fossil forms were free. The 

 attachment takes place very often by means of the ped- 

 uncle ; this is a cylindrical process, sometimes long, 

 sometimes short, connected with the mantle, and passing 

 either through the foramen, or between the apices of the 

 valves. The peduncle consists of horny material, with at 

 times some muscular layers also. Some forms (e.g. 

 Strophalosia) are fixed by spines given off from the surface 

 of the shell ; others (e.g. Crania) by means of one valve 

 which is soldered to the rock or other object. 



The two valves of the brachiopod can be opened and 

 closed by means of muscles (fig. 29), those which open them 

 are called the divaricators (cc'), those which close them, 

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

 been removed the places where the muscles were attached 

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



