The position appears to me to be this: the dermal cranial skeleton 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, _i.e._ 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 palæostracan 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 Ammocoetes and let us see what clue there is to be found there. The skin of Ammocoetes 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 {347}secretion of the underlying cells passes, as is seen in Fig. 140, A and B. This cuticle corresponds to the chitinous covering of the arthropod, and like it is perforated with canaliculi, and, according to Lwoff, possibly contains chitin. The epidermal cells rest on a thick layer of most striking appearance (Fig. 141), for it resembles, in an extraordinary degree, when examined superficially, a layer of chitin; it is called the laminated layer, and is characterized by the extreme regularity of the laminæ. This appearance is due, as the observations of Miss Alcock show, to alternate layers of connective tissue fibres arranged at right angles to each other, each fibre running a straight course and possessing its own nucleus. Although the fibres in each layer are packed close together, they are sufficiently apart to form with the fibres of the alternate layers a meshwork rather than a homogeneous structure, and thus the surface view of this layer shows a regular network of very fine spaces through which nerve-fibres and fluid pass. This layer is easily dissolved in a solution of hypochlorite of soda, a fluid which dissolves chitin. Any one looking at Ammocoetes would say that the only part of its skin which resembles chitin is this laminated layer, and therefore the only part of its skin which would afford an indication of the nature of the skeleton of Pteraspis is this laminated layer, which belongs to the cutis, and not to the cuticle. Yet another significant peculiarity of this layer is its entire disappearance at transformation. Miss Alcock, in a research not yet published, has shown that this layer is completely broken up and absorbed at transformation; the cutis of Petromyzon is formed entirely anew, and no longer presents any