The Holoptychius ranks among the family of Cœlacanthes, and the term Holoptychius (holos, entire; and ptyche, a wrinkle) is applied to the fossil from the circumstance of the scales being covered with wrinkled dots or markings, the enameled surface of which is indented with deep undulating furrows. Another characteristic feature of this genus consists in the distant position of the ventrical fins, being considerably removed toward the tail, and in the arrangement of the branchial organs, which form two large plates between the branches of the inferior ray, as in the genus Megalichthys. The structure of the “nageoirs,” the rounded form of the ventrical fin, and the manner in which the rays of its anterior edge are insensibly prolonged, in connection with their relative thinness, are also marked distinctions. The head of the Holoptychius is remarkably small in comparison with the size of the body, which, in the Clashbennie specimen measures thirty inches in length by twelve in breadth. The scales are still disproportionately larger than either the head or body, some of them being nearly three inches in length by two and a half in breadth, with a corresponding thickness. The structure of the dermal covering is beautiful in the extreme; it is composed of these scale-plates, articulating, and laced together in such a way as to combine the greatest possible strength with the highest degree of flexibility; and, protected by a rich coating of enamel, it must have been capable of the greatest endurance, and of resisting any pressure. Two thickly set rows of teeth; one inner, and extremely minute, the other large and pointed, completed the equipments of a mouth adapted to seize and crush to powder any intruder upon its pasturage. The vertebral column extended to the extremity of the tail, which was forked or divided into two unequal lobes, a contrivance of nature that enabled the animal to turn quickly on its back before striking its prey. This form of the tail is called the HETEROCERCAL; it is characteristic of most of the fishes of the period, and prevailed during the palæozoic age; when it gave way, at the era of the chalk formation, to what is termed the HOMOCERCAL structure, and which still exists in the fishes of the current epoch.