DAVIDSON SCOTTISH CARBONIFEROUS BRACHIOPODA- 



229 



tlie animal itself iu a memoir exclusiyely devoted to fossil species. ITe vill 

 therefore briefly place before tlie reader a few details oiilv concerning tbose 

 muscles wMch have left recognizable impressions in the interior of the valves. 

 It must also be observed that although Mr, Woodward, Mr. Hancock, and Dr. 

 Gratiolet and others agree as to the shape and position of the various muscles, they 

 do not interpret the functions of some of these exactly in the same manner, and as a 

 number of names have been applied to designate the same muscle, the first 

 thin^ to do will be to place before the reader figures showmg the position of 

 the impressions and the synonymous terms that have been employed, and such 

 ^VLll also save us the necessity of describing these scars, which a glance at the 

 figures will explain far better than could be done by simple words. 



The muscular system is much more complex in the unarticulated divisions 

 of the Brachiopoda than in articulated groups ; while describing theii- shape 

 and direction we cannot do better than to follow what has been said by Mr. 

 Hancock in his admirable memoir upon the " xinatomy of the Brachiopoda," 

 published in the " Pliilosophical Transactions of the Boyal Society,"' 1S5S. 



The anterior occlv.sors " are a pair of stout muscles, of about equ:,l thickness 

 throughout ; they pass from the ventral valve, one at each side, in front of 

 the \usceral mass, and inclining forwards and inwards, they go to be attached 

 to the sides of the central ridge of the dorsal valve, about one-third the 

 length of the shell from the anterior margin. The dorsal extremities are com- 

 pressed, and have their sides in contact." 



The posterior occlv.sors " are rather stouter and much stronger, and go dnectly 

 from valve to valve, parallel with each other. The ventral extremities are 

 placed a little in advance of the corresponding terminations of the anterior 

 pair ; and the dorsal extremities of the former are situated a little behind those 

 of the latter." 



The clkaricator, " though forming a single mass, is really two muscles com- 

 bined. It is short and stout, and is situated at the posterior extremity of the 

 perivisceral chamber, passing directly between the valves, and having its 

 attachments immediately within the umbones. The extremities have a semi- 

 circular form, arched behind, and slightly bifid in front, indicating its double 

 natui'e." 



The central adjnstors. — " This pair are attached to the ventral valve by fine 

 points between the posterior occlusors 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 parieties of the body." 



The external adjnstors " 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 outwards and backwards they enlarge a little, and ascending, are 

 inserted into the dorsal valve, one on each side, immediately behind the central 

 pair." 



'posterior adjnstors "are large and powerful muscles, and though they 

 may be considered as a paii', they are asymmetrical, there being two on one 

 side and only one on the other. As they pass across from valve to valve they in- 

 tersect each other, the single one passing between the other two. The single one 

 is as large as the other two put together, and is attached to the left side of the 

 ventral valve, about midway between the divaricator and the anterior occlusor. 

 Erom 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 adjusters 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 



