DAVIDSON — S 
•SCOTTieH CARBONIFEROUS BBACHIOPODA 
220 
ihc animal iiself in a mcinoiv exchisivcly devoted to fossil species. We will 
therefore brielly place before the reader a few details only couceming those 
muscles which 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 ui the same manner, and as a 
number of names have been applied to designate the same muscle, the first 
thing to do will be to place before the reader figures showing the position of 
the nnprcssions and the synonymous terms that have been employed, and such 
will 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 unartieulatcd divisions 
of the Brachiopoda than in articulated groups ; while describing their shape 
and direction we cannot do better than to follow what has been said by Mr. 
Hancock in his admirable memoir upon the " Anatomy of the Brachiopoda," 
published in the "Philosophical Transactions of the Royal Society," 1858. 
The anterior occlusors " are a pair of stout muscles, of about equal thickness 
throughout ; they pass from the ventral valve, one at each side, in front of 
the visceral 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 occlusors " are rather stouter and much stronger, and go directly 
from valve to valve, parallel with eaeli 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 dicaricator, " 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 lunbones. The extremities have a semi- 
circular form, arched behind, and slightly bifid in front, indicating its double 
nature." 
The central adjustors. — " 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 mner 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 adjustors "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 
insertecl into the dorsal valve, oir on each side, immediately behind the central 
pair." 
The posterior adjustors " 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 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 oeclusor. 
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 
