Next to the _Pterichthys_ of the Lower Old Red I shall place its contemporary the _Coccosteus_ of Agassiz, a fish which, in some respects, must have somewhat resembled it. Both were covered with an armor of thickly tubercled bony plates, and both furnished with a vertebrated tail. The plates of the one, when found lying detached in the rock, can scarcely be distinguished from those of the other: there are the same marks, as in the plates of the tortoise, of accessions of growth at the edges--the same cancellated bony structure within, the same kind of tubercles without. The forms of the creatures themselves, however, were essentially different. I have compared the figure of the _Pterichthys_, as shown in some of my better specimens, to that of a man with the head cut off at the shoulders, one of the legs also wanting, and the arms spread to the full. The figure of the _Coccosteus_ I would compare to a boy's kite. (See Plate III., fig. 1.) There is a rounded head, a triangular body, a long tail attached to the apex of the triangle, and arms thin and rounded where they attach to the body, and spreading out towards their termination like the ancient one-sided shovel which we see sculptured on old tombstones, or the rudder of an ancient galley.[K] The manner in which the plates are arranged on the head is peculiarly beautiful; but I am afraid I cannot adequately describe them. A ring of plates, like the ring-stones of an arch, runs along what may be called the hoop of the kite; the form of the keystone-plate is perfect; the shapes of the others are elegantly varied, as if for ornament; and what would be otherwise the opening of the arch, is filled up with one large plate, of an outline singularly elegant. A single plate, still larger than any of the others, covers the greater part of the creature's triangular body, to the shape of which it nearly conforms. It rises saddle-wise towards the centre: on the ridge there is a longitudinal groove ending in a perforation, a little over the apex, (Plate III., fig. 2;) two small lateral plates on either side fill up the base of the angle; and the long tail, with its numerous vertebral joints, terminates the figure. [Footnote K: I have since ascertained that these seeming arms or paddles were simply plates of a peculiar form.]