346 THE ORIGIN OF VERTEBRATES 
looked upon as similar to that of the pulp and dentine canals of 
many fish-scales. 
On the other hand, this dermal covering of Pteraspis has heen 
compared by Patten with the arrangement of the chitinous structure 
of certain parts of the external covering of Limulus, a comparison 
which to my mind presents a great difficulty. The chitin-layers in 
Limulus are external to the epidermal cells, being formed by them ; 
the layers in Pteraspis which look like chitin must have been internal 
to the epidermal layer; for each vascular canal which passes from 
a pulp-cavity on its way to be distributed into the dentine canals 
of the ridge gives off short side branches, which open directly 
into the groove between the ridges. If these canals were filled with 
blood they could not possibly open directly into the open grooves 
between the ridges ; these openings must, therefore, have been covered 
over with an epithelial layer which covered over the surface of the 
animal, and consequently the chitin-like structure must have been 
internal to the epidermis, and not external, as on Patten’s view. 
The comparison of this structure with the dentine of fish-scales 
signifies the same thing, for in the latter the epidermis is external 
to the dentine-plates, the hard skeletal structure is in the position 
of the cutis, not of the cuticle. 
The position appears to me to be this: the dermal cranial skele- 
ton of vertebrates, whether it takes the form of a bony skull or of 
the dorsal plates of a cephalaspid or a pteraspid is, in all cases, not 
cuticular, 7.c. is not an external formation of the epidermal cells, but 
is formed in tissue of the nature of connective tissue underlying the 
epidermis. On the contrary, the hard part of the head-carapace of 
the paleostracan is an external formation of the epidermal cells. 
If, then, this tissue of Pteraspis is not to be looked upon as 
chitin, how can we imagine its formation? It is certainly not bone, 
for there are no bone-corpuscles; it is a very regular laminated 
structure resembling in appearance chitin rather than anything else. 
As in all cases of difficulty, turn to Ammoccetes and let us see 
what clue there is to be found there. The skin of Ammoccetes is 
peculiar among vertebrates in many respects. It consists of a number 
of epidermal cells, as in Fig. 140, the varying function of which 
need not be considered here, covered over with a cuticular layer 
which is extraordinarily thick for the cuticle of a vertebrate skin ; this 
cuticular layer is perforated with fine canaliculi, through which the 
