556 
LOWEST VERTEBRATES . 
Cephalaspis. 
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 evidence 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 
truncated behind, and with the rounded margin bent inwards below to form an 
ornamented 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 fins 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 
Pterichthys. . . & 1 ’ x J 
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 
restoration op Cephalaspis. (From Woodward, Cat. Foss. Fish. Brit. Mus.) 
the head-shield and the united scales of the body. The head is, however, sharply 
defined from the body; and the armour, 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 append¬ 
ages, jointed near the middle. A 
small movable plate between the 
eyes seems to have lodged a median 
eye, anothei movable plate on the restoration of pterichthys. (From Traquair.) 
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 
definite is known with regard to the nature of the nose, mouth, and jaws. 
