MUSCULATURE. 93 



taclnnent are so disposed as to indicate longitudinal flexor and extensor muscles rather than 

 short muscles extending from segment to segment. Indeed, the tenuity of the ventral mem- 

 brane is such as to preclude the possibility of enrollment by the use of muscles of that sort, 

 while powerful longitudinal flexors could have been anchored to cephalon and pygidium. The 

 strongly marked character of the neck-ring of trilobites is probably to be explained as due 



Fig. 20. Restoration of a part of the internal organs of 

 Ceraurus pleurexanthemus as seen from above. At the sides 

 are the extensor muscles, and in the middle, the heart overlying 

 the alimentary canal. Drawn by Doctor Elvira Wood, under 

 the supervision of the author. 



to the attachment of the extensor muscles, rather than to its recent incorporation in the 

 cephalon. The same is true of the anterior ring on the pygidium. 



Possible preservations of extensors and flexors in Ceraurus. Among Doctor Walcott's 

 sections are four slices which I should not like to use in proving the presence of longitudinal 

 muscles, but which may be admitted as corroborative evidence. Two of these transverse 

 sections (Nos. 114 and 199) show a dorsal and a ventral pair of dark spots in positions 

 which suggest that they represent the location of the dorsal and ventral muscles, while two 

 others (Nos. 131 and 140) show only the upper pair of spots. The chief objection to this 



