326 GENERAL SUMMARY TO 



angles to the plane of the ventral valve and close it instantaneously like in the lid of a 

 snuff-box. 



In his valuable memoir, 1 this distinguished French zoologist gives the most complete 

 description of the animal of Thecidium we now possess ; he says " that there are three pairs 

 of muscles possessing distinct directions and functions, six muscles in all. Of these six 

 muscles two are lateral, short, wide, and easily observable, and serve, without doubt, for 

 the occlusion of the shell." (These are Duthiers' " muscles adducteurs lateraux ou 

 externes.") Hancock informed me by letter that these would represent his " adjusters," 

 but that the function of this pair of muscles does not appear to be exactly similar to the 

 " adjusters " of Waldheimia, in which they move the shell upon the peduncle ; for in 

 Thecidium they assist both in closing the valves and in preventing any irregular or 

 lateral movements, which, from the central position of the occlusors, might be liable to 

 take place. In fact, he believed that the functions of the " adjusters " in Thecidium is 

 modified, much as it is in the same muscles of Lingula, and he was therefore inclined to 

 conjecture that the two valves of the former are not so firmly hinged as they are in other 

 articulated forms. "The second pair are those nearest to the median line." These are 

 termed the " muscles adducteurs internes " by Professor Lacaze-Duthiers, and are 

 equivalent to Hancock's " anterior and posterior occlusors." In the ventral valve 

 these muscles are fixed to the bottom of the valve nearly under the extremity of 

 the tongue-shaped "ascending shelly process," and then attached to the two thin 

 shelly processes which we have described as existing in the cavity of the beak. 

 The functions of these muscles (as stated by Duthiers) is also to effect a closing 

 of the valves, but less efficaciously than the preceding pair. The third pair, which 

 M. Duthiers designates " muscles adducteurs," are Hancock's " divaricators," and 

 their function is to open the valves. They form four large impressions on the 

 bottom of the ventral valve, on either side of the central septum, and have their other 

 end attached to the extremity of the cardinal process of the dorsal valve. 



Although the characters relating to the disposition of the muscles and their functions 

 in the Tretenterata and Clistenterata are now fairly ascertained, it would be very desir- 

 able that they should be described and figured in those recent genera that have not been 

 yet anatomically examined. Their characters are of very great importance to the 

 palaeontologist in his study of the extinct genera and species, and I have devoted much 

 time and attention to the subject during my many researches in connection with both 

 recent and fossil forms. 



In the Tretenterata, of which Lingula and Discina may be quoted as examples, the 

 myology is much more complicated, and anatomists have differed considerably in their 

 respective views concerning the functions of some of the muscles. They have been care- 



1 " Histoire naturelle des Brachiopocles vivants de la Mediterranee," ' Annales des Sciences Naturelles," 

 4th ser., vol. xv, p. 258, 1861. 



