Report of the State Geologist. 



505 



In Lingula and its allies the muscles are more numerous and 

 their arrangement Ear more complicated 



than in other brachiopods. These ani- 

 mals possess the power of separating the 



valves in a manner unlike that of other 

 members of the class ; that is, of sliding 

 them apart laterally or rotating them 

 through a short arc about the longitudinal 

 axis of the shell. The muscular area 

 occupies a relatively large portion of the 

 internal surface, the bands being arranged 

 about the margins of the visceral area. 

 In the umbonal region there is a central, 

 undivided muscle, crossing the interior 

 cavity vertically ; this is the umbonal or 

 diductor muscle, by its contraction open- 

 ing the valves along their anterior 

 margins. 



The adductors (or centrals) are a single 

 pair situated near the anterior extremity 

 of the area, and they also cross the shell 

 cavity vertically. At the anterior ex- 

 tremity of the area in the pedicle 

 valve originates a single pair of protractors {middles), each 

 member of which extends backward and is inserted near the 

 lateral margin of the brachial valve, while a second pair of 

 protractors {externals) originates just behind the adductors of the 

 pedicle-valve and is inserted behind the first pair. These muscles, 

 by their combined action, or independently, draw the brachial 

 valve forward. The retractors (or anteriors), or those which 

 readjust the brachial valve, extend from the outer lateral margins 

 of the area in the pedicle-valve to its anterior extremity in the 

 brachial valve. Neither protractors nor retractors cross the peri- 

 visceral cavity. The rotators (transmedian or sliding-muscles) are 

 three in number, two on one side and three on the other. These 

 are situated posteriorly and cross the perivisceral cavity diago- 

 nally, the undivided muscle passing between the other two. The 

 contraction of these, alternately rotates the brachial valve, first in 

 one direction and then in the other. In Lingula, also, the paries 

 1891. 64 57 



Fig. 120. — Muscular system of 

 Lingula anatina: c, alimen- 

 tary canal; d, dorsal vesicle; 

 ef, walls of perivisceral cavity ; 

 g, retractors; h, adductors; 

 i, umbonal; j k, protractors; 

 I, rotators: m, pedicle; n, 

 pedicle muscle.— CHancock). 



