202 BRITISH CARBONIFEROUS BRACHIOPODA. 



The central adjusters. — " This pair arc attached to the ventral valve by fine points 

 between the posterior occlnsors in front ; they are placed close together, one on each side 

 of the median line. Sweeping round the inner border of these muscles, they diverge 

 posteriorly, and increasing in size as they go, ascend towards the dorsal valve, to which 

 they become adherent, one on each side, immediately within the parietes of the body." 



The external adjusters " arise from the ventral valve, at the outside of the posterior 

 occlusors, and in contact with them. They are at first pretty stout, but on passing out- 

 wards and backwards they enlarge a little, and ascending, are inserted into the dorsal 

 valve, one on each side, immediately behind the central pair." 



The posterior adjusters " are large and powerful muscles, and though they may be 

 considered as a pair, they are asymmetrical, there being two on one side and only one on 

 the other. As they pass across from valve to valve they intersect each other, the single 

 one passing between the other two. The single one is as large as the other two both 

 together, and is attached to the left side of the ventral valve, about midway between the 

 divaricator and the anterior occlusor. From this point it passes diagonally upwards and 

 forwards. . .and on reaching the opposite side of the dorsal valve has the other end inserted 

 into the latter, immediately within the posterior terminations of the external and central 

 adjustors of the same side. At the points of attachment the three muscles are pressed so 

 close together that they appear at first sight as only one. The two opposite posterior 

 adjustors take their origin from the right side of the ventral valve, considerably apart ; 

 but both of them close to the lateral parietes of the body, one only a little in advance of 

 the divaricator, and the other a short distance further forward. They converge as they 

 penetrate the visceral mass, and sloping forward, one on each side of the visceral muscle, 

 with the alimentary tube above them, they ascend to their insertion into the left side of 

 the dorsal valve, directly within those of the external and central adjustors. Therefore at 

 this point there are the terminations of four muscles in close contact." 



The peduncular muscle "has its insertion immediately within the umbo of the ventral 

 valve, and close behind the divaricator." 



x is a line indicating attachment of the posterior parietals. 



There are some other muscles, but as they do not leave any impressions upon the 

 surface of the shell, will not require to be recorded here ; but now that the reader has had 

 the names, shape, and direction of the muscles explained, it will be necessary to mention 

 as briefly as possible what are their supposed functions, and for this purpose I will 

 particularly mention the views of Mr. Hancock and Dr. Gratiolet, as they are the authors 

 who have more recently examined the animal with the greatest attention. In order to 

 avoid the possibility of error upon my part, I requested the last-named two anatomists to 

 kindly transmit me their views, which I will now transcribe. 



According to Mr. Hancock, the functions of the various muscles might be shortly 

 described as follows : 



" The anterior and posterior occlusors are mainly instrumental in closing the valves. 



