The body of the megalichthys was cased in an armature of as solid and glittering bone as that which defended its head. Where the plates of the cranium ended off they were succeeded by large rhomboidal scales that crossed the body obliquely, and overlapped each other like the metal plates in the antique scale-armour. Each scale consisted of two parts, of which one had a rhomboid form and was covered over with enamel, while the other ran round the two inner sides of the rhomb as a broad unenamelled selvage deeply indented along its centre. It was the enamelled portion alone that formed the outer surface, the rough unpolished border being covered by the overlapping edges of the adjoining scales. The scales had not a uniform thickness, but were strongest at the covered part from which each thinned off to the outer edges. In this way the thin edge of one scale pressed down on the thick part of the subjacent one, and a covering of uniform strength and smoothness was produced. Looking at a set of these scales as they still occupy their original position on the creature's body, it is scarcely more than a half of each which meets our eye; for the unenamelled border occupied about a third of the entire surface, and a fourth of the remainder was covered by the overlapping scales. The effect of this arrangement must have been to combine great strength with the most perfect flexibility. Notwithstanding the bulk of his helmet and the weight of his scale-armour, we cannot conceive the megalichthys to have been other than a lithe, active, predaceous fish, dealing death and destruction among the herring-like shoals of little palæonisci and amblypteri, though able to maintain perhaps but a doubtful warfare with his more bulky contemporary, the holoptychius. The internal structure of the scales of the megalichthys exhibits the same provision for combining strength with the least possible amount of material. Viewed in a transverse section under a magnifying power of about eight diameters, they are seen to consist of three layers of bone; each possessing a peculiar structure. The outermost is formed of a tessellated pavement of minute round ocelli, having a fine brown colour, and placed close together with considerable regularity. They somewhat resemble little wheels, the axle being either a dark solid nucleus or a small circular aperture, whence there radiates to the outer rim a set of exceedingly minute fibres which were originally hollow, and served as canals to carry on the growth of the scale. The vacant space left where four wheels impinge on each other, forms one of the pores that cover the enamelled surface of the scale. The whole structure of this outer layer very closely resembles that presented by the internal part of the base of the teeth, save that the confluent lobes shown in the teeth become in the scale detached into separate and independent circles. The central stratum of each scale is composed of