4i6 



THE ORIGIN OF VERTEBRATES 



bending or fracture of the calcified plates would take place along 

 this line. There is, undoubtedly, an appearance of finish at the 

 termination of these skeletal friuges, as though they terminated in 

 a definitely shaped spear-like point, just as is seen in the trilobite 

 pleura?. This, again, to my mind, is rather evidence of pleural fringes 

 than of true appendages. 



As already argued, I look upon Ammoccetes as the only living 

 fish at all resembling the cephalaspids ; it is therefore instructive to 

 compare the arrangement of this spinal dermo-septal skeleton of 

 Cephalaspis with that of the septa between the myomeres in the 



B 



Fig. 162. — A, Arrangement of Septa in Ammoccetes (NC, position of notochord) ; 

 B, Arrangement op Septa in Amphioxus. 



trunk-region of Ammoccetes and Amphioxus. Such a skeleton in 

 Ammoccetes would be represented by a series of plates overlapping 

 each other, arranged as in Fig. 162, A, and in Amphioxus as in 

 Fig. 162, B. I have lettered the corresponding parts of the two 

 structures by similar letters, a, b, c. Ammoco^tes differs in configu- 

 ration from Amphioxus in that it possesses an extra dorsal (a, d) and 

 an extra ventral bend. Ammoccetes is a much rounder animal than 

 Amphioxus, and both the dorsal and ventral bends are on the extreme 

 ventral and dorsal surfaces — surfaces which can hardly be said to 

 exist in Amphioxus. The part, then, of such an aponeurotic skeleton 



