r 68 ] 



for certfiiu nnimpoi-tant modifications observable in the shells com- 

 posing the different families and genera thereof. In the Tretenterata, 

 of which Lingula might be quoted as an exaoiple, the myology was 

 much moi'e complicated, and anatomists had differed considerably 

 in their respective views concerning the function of some of the 

 muscles. It would not, however, be possible to discuss the opinions 

 of Hancock, Vogt, Gratiolet, and others, but he would refer to 

 and adopt those recently advocated by Professor Kiug, as 

 they seemed to carry with them a greater degree of jjlausibility. 

 Of shell or valvular muscles. Professor King, made out five pairs 

 and an odd one. Three pairs were laterals having their members 

 limited to the sides of the shell; one pair were transmedians, each 

 member passing across the middle to reverse side of the shell, one 

 pair had its members confined to the central region ; while the odd 

 muscle occupied the umbonal cavity, and he assumed that the 

 ccnti-al and umbonal muscles effected. the direct opening and closing 

 of the shell ; the laterals enabled tlie valves to move forward and 

 backward on each other, while the transmedians allowed 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 was in the 

 medio transversed 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 matter had been set at rest by 

 Pi'ofessors Semper and Morse, who actually observed the animal 

 perform the operation; they mention that it was never done 

 suddenly or by jerks, as the valves were at 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 rested 

 opposite to each other and widely apart. Those who had not seen the 

 animal in life, or who did not believe in the possibility of the valves 

 ci'ossing each other with a slight obliquity, would not consent to 

 appi'opriating 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. They 

 had not only the observations of Semper and Morse, but the 

 anatomical investigations of King, to confirm the slidiug action or 

 divarication of the valve of Lingula. In the clidenterata, where no 

 such sliding action of the valves was necessary or j^ossible, no 

 muscles for such an object were required ; consequently none took 

 rise from the lateral portions of the valves as in Lingula, but, in an 



