a loose open network of cancellated bone that passes into the layer on either side, and resembles in its general texture the osseous vertebræ of the same fish. The under layer, one end of which rested immediately on the skin, approaches more to the firmness and solidity of the outer one, but, in place of a tessellated, ivory-like pavement, it had a close fibrous texture, with here and there a scattered cavity, and the fibres were matted together so as to resemble the more solid structure of the cranial bones. The effect of this triple arrangement must have been to impart great strength and lightness to the external armature of the fish; the middle spongy layer serving, by its porosity, at once to deaden the effect of any blow aimed at the outside, and to give buoyancy and lightness to what would otherwise have been a coat of mail well-nigh, as ponderous as that of a feudal chief. One can hardly conceive any implement of warfare in use among the lower animals of strength enough to pierce this massive covering. But we shall find as we go on that if the megalichthys had a strong defensive armour, a bulkier neighbour had a still stronger offensive one, and that the enamelled plates of the one fish were scarcely a match for the huge pointed tusks of the other. The megalichthys had an osseous skeleton, with vertebræ of a discoidal form. These internal bones when viewed under the microscope are found to display an open cancellated structure, resembling that of the central layer in the scales. It thus appears that this ancient fish was not merely defended by a hard external armour, but possessed an equally solid framework of bone within. Mingled with the scales and bones of the megalichthys, there are found the remains of a still larger fish, to which the name of Holoptychius has been given. Its external ornament differed entirely from that of the animal last described. It possessed teeth sometimes six or seven times larger, and jaws, plates, and bones of a form and dimensions totally distinct. Strange as it may seem, however, these two fishes have been constantly and systematically confounded from the time when they were first discovered. Two or three years ago, there might be seen in the British Museum several specimens of the holoptychius, of which some bore the correct name, while the rest were labelled "Megalichthys;" and a similar error prevailed in several of the other museums.[44] The confusion can be traced very distinctly in the memoir of Dr. Hibbert, who for the first time described the remains of these fishes, and wrote according to information received from Agassiz. [Footnote 44: The mistake was noticed in 1845 by Hugh Miller, who, in a foot-note to his _First Impressions of England and its People_, p. 71, well defines the distinctions between the two ichthyolites.]