io PALEONTOLOGY 



process) from the dorsal valve into the umbonal cavity of 

 the ventral valve. The line along which the margins of 

 the two valves meet in the neighbourhood of the hinge- 

 teeth is called the hinge-line. To effect the opening of 

 the valves there are muscles fixed at one "end to the 

 cardinal process, at the other to the interior of the 

 ventral valve (divaricator muscles) ; to close the valves 

 there are other muscles running across from valve to 

 valve (adductor muscles). All muscles act by contraction 

 which pulls their two attachments towards one another. 

 The dorsal attachments of these two sets of muscles 

 being on opposite sides of the fulcrum (the hinge-line), 

 they work against one another. The muscles of the 

 pedicle are attached to the interior of the ventral valve. 



When the muscles decay after death, the areas of the 

 shell to which they were attached show as slight depres- 

 sions devoid of the smoothness of the rest of the internal 

 surface. Thus it is possible to tell in a fossil brachiopod 

 the presence and arrangement of these muscles. Although 

 the details differ greatly among the many forms of 

 brachiopods, the adductor impressions in the ventral 

 valve are always close together, and the divaricator im- 

 pressions lie more or less to the right and left of them.. 



The body proper of a brachiopod being so small, it is 

 not surprising that the reproductive organs should extend 

 into the mantle which lines the two valves ; impressions 

 of the ovaries are in many forms to be seen on the inner 

 surface of the shell ; so also are the branching impres- 

 sions of vessels belonging to a circulatory system 

 (vascular markings'). In forms like the terebratuloids 



