230 
THE GEOLOGIST. 
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 tlie body, one only a little ia 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 single muscle, with 
the alimentary tube above them, tliey ascend to their iusertiou 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 con- 
tact." 
The peduncular muscle " has its insertion immediately within the umbo of tlie 
ventral valve, and close behind the divaricator." 
.r 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 here recorded ; but now that the 
reader has had the names, shape, and position of the muscles explained, it will 
be necessary to mention as briefly as possible what are their supposed functions, 
and for this purpose we 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 two last named anatomists to kindly transmit 
me their views, and which I wiU now transcribe. 
According to Mr. Hancock, the functions of the various muscles might be 
shortly described as follows : — 
" The anterior and jmsterior occlusors are mainly instrumental in closing the 
valves. 
" The dkaricators are the chief agents in opening them. TVTien they con- 
tract the umbonal regions of the valves are approximated, and thus pressing 
forward the fluid in the perivisceral chamber, their anterior margin is separated. 
" The primary function of the three pairs of adjustors is to keep the valves 
opposed to each other, or, in other words, to adjust them; and in this respect 
to compensate for the deficiency of a hinge and condyles. Wlieu in full action 
and in co-operation with the oeclusors and divaricator, they likewise assist in 
closing the valves. The adjustors are the sliding muscles of those authors who 
believe in the sliding of the valves over each other. The anterior oeclusors 
have had a similar function assigned to them. 
The peduncular muscle attaches the sheU to the peduncle, and has probably 
the power of moving the former upon the latter." 
Some doubt exists concerning the homology of the adjust or muscles (in 
Lingula) ; but Mr. Hancock lias not expressed any strong opinion on the pomt, 
he thought it likely that the muscles so named in the articulated and uii- 
articulated genera of Brachiopoda were probably homologous, but is ready to 
admit that he may be possibly mistaken, and, if so, he would not however be 
disposed to change the names, for in both di\'isions the function of these muscles 
is to adjust the valves. It is only necessary to keep in view that they are not 
homologous. 
We will now give Dr. Gratiolet's description of the functions of the muscles, 
and for which I am indebted to the author himself, who has kindly therein dis- 
tinguished the effects of simultaneous action and those of the alternative action; 
and for the assistance of the reader Mr. Hancock's names have been added 
within brackets. 
1. — MuscLKs Preadducteuus [Posterior Adductor] and Post-Adduc- 
TEUKS [DiVARICATORS]. 
a. Simultaneous action. — The energetically drawing together the valves in 
their whole length. 
