416 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 fringes, as though they terminated in 
a definitely shaped spear-like point, just as is seen in the trilobite 
pleure, 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 
Fic. 162.—A, ARRANGEMENT OF SEPTA In AmmocasTES (NC., position of notochord) ; 
B, ARRANGEMENT OF 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. Ammoccetes 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 
