THE BRITISH FOSSIL BRACHIOPODA. 



327 



fully studied and illustrated by Owen, Vogt, Hancock, Gratiolet, Semper, 1 Woodward, 

 Morse, Brooks, and others ; and more recently by King, 3 whose views seem to carry with 

 them a greater degree of probability ; and 

 these opinions have been confirmed by 

 Brooks in his admirable memoir on 

 Lingula? Of the muscles of the shell 

 (valves) he makes out five pairs and an 

 odd one, and individualises their respec- 

 tive functions as follows : — Three pairs 

 are lateral, having their members limited 

 to the sides of the shell ; one pair is 

 transmedian, each member passing across 

 the middle of the reverse side of the 

 shell, while the odd muscle occupies the 

 umbonal cavity. The central or umbonal 

 muscles effect the direct opening and 

 closing of the shell ; the laterals enable 

 the valves to move forwards and 

 backwards on each other. The trans- 

 medians allow the similar extremities 

 (the rostral) of the valves to turn from 

 each other to the right or the left on an 



axis subcentrically situated, that is, the medio-transverse region of the dorsal valve. It was 

 long a matter in discussion whether the animal could displace its valves sideways when 

 about to open its shell, but this has been actually observed by Professors Semper, Morse, 

 and Brooks, who saw the animal perform the operation. They mention that it is never 

 opened suddenly or by jerks, as the valves are first always pushed to one side several times 

 and back again on each other, at the same time opening gradually in the transverse 

 direction till they rest opposite to one another and widely apart. Those who have not 

 seen the animal in life, or did not believe in the possibility of the valves crossing each 

 other with a slight obliquity, would not consent to appropriating any of its muscles to 

 that purpose, and consequently attributed to all the lateral muscles the simple function 

 of keeping the valves in an opposite position, or holding them adjusted. 



We have not only the observations of Semper and Morse, but the anatomical investi- 

 gations of King to confirm the sliding action or lateral divarication of the valves of Lingula. 

 In the Clistenterata, where no such sliding action of the valves is necessary or possible, 



Lingula anatina (after King). 



Pig. A. Interior of the ventral valve. 



Fig. B. Interior of the dorsal valve g, umbonal muscular 

 impressions (open valves) ; h, central muscles (close valves) ; 

 i, transmedial or sliding 1 muscles ; b, parietal band ; j, Tc, I, 

 lateral muscles (J, anteriors ; Tc, middles ; I, outsiders), en- 

 abling the valves to move forward and backv 

 other. 



jrd on each 



1873. 



'Zeitschrift fur wissenschaftliche Zool.,' vol. ii, p. 100, 1859, and vol. xiv, p. 424, 1864. 



" On some Characters of Linyula anatina," ' Ann. and Mag. of Nat. History,' 4th ser., vol. xii, 



" The Development of Lingula, &c," ' Chesapeake Zool. Laboratory, Scientific Results,' p. 35, 18/9. 



