230 



THE GEOLOGIST. 



one. The two opposite posterior adjustors take theii' 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 single muscle, with 

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

 of the dorsal Valve, dnectly within those of the external and central adjustors. 

 Therefore at this point there are the terminations of four muscles in close con- 

 tact." 



The pedmiadar muscle " lias its insertion immediately within the umbo of the 

 ventral valve, and close behind the divaricator." 



a- 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 wiU 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 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. 



" The dimricators are the chief agents in opening them. T^lien 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 occlusors 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 occlusors 

 have had a similar function assigned to them. 



The peduncular muscle attaches the shell to the peduncle, and has probably 

 the power of moving the former upon the latter," 



Some doubt exists concerning the homology of the adjuster muscles (ui 

 Lingula) ; but Mr. Hancock has not expressed any strong opinion on the point, 

 he thought it likely that the muscles so named in the articulated and un- 

 articulated genera of Brachiopoda were probably liomologous, 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 divisions 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 fiuictions of the muscles, 

 and for wliicli I am indebted to the autlior himself, who has kindly therein dis- 

 tinguished the cfl'ects of simultaneous action and those of the alternative action ; 

 and for tlio assistance of the reader Mr. Hancock's names have been added 

 within brackets. 



1. — Muscles Peeadducteites [Posteeioe Addittor] axd Post-Adduc- 



TELiES [DiVAEICATOES]. 



a, Sinndtaneoinf action. — The energcticallv drawing together the valves in 

 tlicir wliolc lengt h. 



