regular laminated character, but resembles rather the sub-epidermal connective tissue layer of the skin of higher vertebrates. This laminated layer, then, just like the muco-cartilage, shows, by its complete disappearance at transformation, its ancestral character. [Illustration: FIG. 140.--EPITHELIAL CELLS OF AMMOCOETES TO SHOW THE CANALICULI IN THE THICK CUTICLE (B). A, TRANSVERSE SECTION THROUGH THE CUTICLE.] Very suggestive is the arrangement of the different skeletal {348}tissues in the head-region of Ammocoetes. Fig. 141 represents a section through the head near the pineal eye. Most internally is _a_, a section of the membranous cranium, then comes _b_, the muco-cartilaginous skeleton, then _c_, the laminated layer, and finally _d_, the external cuticle. If in Ammocoetes we possess an epitome of the history of the vertebrate, how would these layers be represented in the past ages, supposing they could be fossilized? [Illustration: FIG. 141.--SECTION OF SKIN AND UNDERLYING TISSUES IN THE HEAD-REGION OF AMMOCOETES. _a_, cranial wall; _b_, muco-cartilage; _c_, laminated layer; _d_, external cuticular layer.] The most internal layer _a_, by the formation of cartilage and then bone, represents the great mass of vertebrate fossils; the next layer _b_, by a process of calcification, as previously argued, represents the head-shield of the Osteostracan fishes; while the cuticular layer _d_, no longer thin, is the remnant of the Palæostracan head-carapace. Between these two layers, _b_ and _d_, lies the laminated layer _c_. Intermediate to the Palæostracan and the Osteostracan comes the Heterostracan, with its peculiar head-shield--a head-shield whose origin is more easily conceivable as arising from something of the nature of the laminated layer than from any other structure represented in Ammocoetes. My present suggestion, then, is this: the transition from the skeletal covering of the Palæostracan to that of the highest vertebrates was brought about by the calcification of successive layers from without inwards, all of which still remain in Ammocoetes and show how the external chitinous covering of the arthropod was gradually replaced by the deep-lying internal bony cranium of the higher vertebrates. In Ammocoetes the layer which represents the covering of the {349}Palæostracan has already almost disappeared. At transformation the layers representing the stage arrived at by the Heterostracan and the Osteostracan disappear; but the stage representing the higher vertebrates, far from disappearing, by the formation of cartilage reaches a higher stage and prepares the way for the ultimate stage of all--the formation of the bony cranium. So much for the evidence as to the nature of the structure of the head-shield of the Pteraspidæ.