"rooting" for these in the mud, or burrowing therein for their safety. In these respects they have a most curious analogy to the Trilobites, which in habits they must have greatly resembled, though belonging by their structure to an entirely different and much higher class. So close is this resemblance, that their head-shields used to be mistaken for those of Trilobites. The case is one of those curious analogies which often occur in nature, and which must always be distinguished from the true affinities which rest on structural resemblances. Another group of small fishes, likewise cuirassed in bony armour of plates, may be represented by the _Pterichthys_, with its two strong bony fins at the sides, which may have served for swimming, but probably also for defence, and for creeping on or shovelling up the mud at the bottom of the sea. But, besides the Ganoids which were armed in plated cuirasses, there were others, active and voracious, clad in shining enamelled scales, like the bony pikes of the American rivers and the _Polypterus_ of the Nile. Some of these, like the _Diplacanthus_, or "double-spine" were of small size, and chiefly remarkable for their sharp defensive bony spines. Others, like _Holoptychius_ (wrinkled-scale) and _Osteolepis_ (bone-scale), were strongly built, and sometimes of great size. One Russian species of _Asterolepis_ (star-scale) is supposed to have been twenty feet in length, and furnished with strong and trenchant teeth in two rows. These great fishes afford a good reason for the spines and armour-plates of the contemporary trilobites and smaller fishes. Just as man has been endeavouring to invent armour impenetrable to shot, for soldiers and for ships, and, on the other hand, shot and shells that can penetrate any armoury so nature has always presented the spectacle of the most perfect defensive apparatus matched with the most perfect weapons for destruction. In the class of fishes, no age of the world is more eminent in these respects then the Devonian.[M] In addition to these fishes, there were others, represented principally by their strong bony spines, which must have been allied to some of the families of modern sharks, most of them, however, probably to that comparatively harmless tribe which, furnished with flat teeth, prey upon shell-fishes. There are other fishes difficult to place in our systems of classification; and among these an eminent example is the huge _Dinichthys_ of Newberry, from the Hamilton group of Ohio. The head of this creature is more then three feet long and eighteen inches broad, with the bones extraordinarily strong and massive. In the upper jaw, in addition to strong teeth, there were in front two huge sabre-shaped tusks or incisors, each nearly a foot long; and corresponding to these in the massive lower jaw were two closely joined conical tusks, fitting between those of the upper jaw. No other fish presents so frightful an apparatus for destruction; and