THE ARMORED PRIMEVAL VERTEBRATES 



2933 



Cephalaspis 



shield probably covered " a truly fish-like head; and if it was not for the invariable 

 absence of limbs and jaws, and the forward position of the breathing apparatus in 

 Pteraspis and its allies, these animals might be placed, without hesitation, in the 

 class of fishes. The possibility that limbs and jaws were present, but not calcified 

 enough to be preserved, must, however, be borne in mind; while the negative evi- 

 dence on this subject, and the want of information as to the nature of the tail, are 

 factors necessitating caution in the determination of affinities." 



The next family of the group is typified by the genus Cephalaspis, 

 in which the front shield appears to be confined to the head and gill 

 region, and consists of a single piece, rounded or pointed in front, abruptly trun- 

 cated behind, and with the rounded margin bent inward below to form an orna- 

 mented flattened rim. Of the triple-layered shield, the inner layer is bony, the 

 thick middle one solid, although traversed by a network of blood vessels, while the 

 upper one is tuberculated and resembles teeth in structure. The eyes are placed 

 close together in the middle of the shield, the nostrils must have had much the 

 same position as in Pteraspis, and at the back of the shield there occurs on each 

 side a small flap which must be regarded as a gill cover. Immediately behind the 

 shield commences the 

 ordinary scaling of 

 the body, without any 

 signs of arches for the 

 support of limbs.- 

 Paired fi n s appear, 

 indeed, to be totally 

 absent, although a dorsal and a caudal fin, stiffened by little elongated scales in 

 place of rays, are present. The large, deep, quadrangular scales covering the body 

 form a series of interlocking rings, doubtless corresponding in the living state to 

 the underlying muscle plates of the body. 



The third modification of the group, as represented by the Devonian 

 Pterichthys, agrees in the general structure of the shield with certain 

 members of the last section in which there is no dividing line between the head 

 shield and the united scales of the body. The head is, however, sharply defined 

 from the bod}'; and the armor, instead of being simple, consists of a number of 

 overlapping plates arranged symmetrically to one another. An important point of 



distinction from all the preceding 

 forms is to be found in the 

 presence of a pair of hollow limb- 

 like pectoral appendages, jointed 

 near the middle. A small mov- 

 able plate between the eyes seems 

 to have lodged a median eye; 

 another movable plate on the 

 cheek appears to represent the 

 gill cover; and a pair of loose jaw plates on the lower surface of the front of the 

 head, in some forms at least, are finely toothed on the hinder border; but nothing 



RESTORATION OF Cephalaspis. 



(From A. S. Woodward, Cat. Foss. Fish. Brit. Mus.) 



RESTORATION OF PTERICHTHYS. 

 (From Traquair.) 



