I Ganoidei. As far as is yet known with certainty, the oldest representatives of the fishes belong to this order. The order is represented, namely, in the Upper Silurian rocks by the remains of at least four genera. In the Devonian rocks, or Old Red Sandstone, the Ganoids attain their maximum both in point of numbers and development. The Placoganoid division of the order is represented by the singular genera Pterichthys, Cephalaspis (fig. 267), Pteraspis, and Coccostens. The Lepidoganoid division of the order is now also abundantly represented for the first time, the genera Dipterus, Osteolepis (fig. 265), Glyptolepis, Holoptychius, Dipla-canthns, and many others, belonging to this section. As regards the further distribution of the Placoganoids, the section of the Ostracostei, characterised by the great development of the cephalic buckler, appears to have died out at the close of the Devonian period. The other section, however - namely, that of the Sturionidae - is represented in the Liassic period (Mesozoic) by the genus Chondrosteus, and in the Eocene (Kaiuozoic) by a true Sturgeon, the Acipenser toliapicus.

The Lepidoganoids continue from the period of the Old Red in great profusion, and they are represented by very many genera in the Carboniferous and Permian rocks. In the earlier portion of the Mesozoic period - i.e., in the Lias and Trias - they are still represented, but all the forms are as yet hetero-cercal. In the Oolitic rocks, for the first time, Lepidoganoids with homocercal tails (fig. 266, B) appear, and they continue to be represented up to the present day.

II. Elasmobranchii. - Like the Ganoidei, the great order of the Sharks and Rays is one of vast antiquity. At the top of the Upper Ludlow rocks, or at the close of the Upper Silurian epoch, there have been discovered the remains of undoubted Plagiostomous fishes, mostly nearly allied to the existing Port Jackson Shark (Cestracion Philippi). These remains consist chiefly of defensive spines, which formed the first rays in the dorsal fins, and upon these the genus Onchus has been founded. Besides these there have been found portions of skin or "shagreen," with little placoid tubercles, like the skin of a living shark. These have been referred to the genus Sphagodus; They are the earliest known remains of Plagiostomous fishes, and, with the exception of the few remains from the Lower Ludlow rocks, they are the earliest known remains of fishes in the stratified series. The discovery of these remains, at that time the earliest known traces of Vertebrate life, is due to the genius of Sir Roderick Murchison, the author of 'Siluria.'

Fig. 275.   1. Spine of Pleuracanthus (one of the Rays); 2. Gyracanthus; 3. Ctena canthus; 4. Tooth of Petalodus; 5. Psammodus; 6. Ctenoptychius. All from the Carboniferous rocks.

Fig. 275. - 1. Spine of Pleuracanthus (one of the Rays); 2. Gyracanthus; 3. Ctena-canthus; 4. Tooth of Petalodus; 5. Psammodus; 6. Ctenoptychius. All from the Carboniferous rocks.

Most of the fossil Elasmobranchii belong to the division Cestraphori of Owen, so called because they are provided with the large fin-spines, which are known to geologists as "ichthyo-dorulites." The two families of this division - the Cestracionts and Hybodonts - are largely represented in past time, the former chiefly in the Palaeozoic period, the latter chiefly in the Mesozoic rocks. Above (fig. 275) is an illustration of the "ichthyodorulites" and teeth of some of the Palaeozoic Cestraphori.

The true Sharks are represented in the later Mesozoic deposits (e.g., by teeth of Notidanus in the Oolites); but they are chiefly Tertiary. The teeth of Odontaspis, Galeocerdo, and Carcharodon, are good examples from the Eocene. The true Rays are older than the true Sharks, the Carboniferous fossil, Pleuracanthus, being probably the spine of a Ray (fig. 275). Numerous remains of Rays, chiefly in the form of the pavement-like teeth, are known, both from the Secondary and Tertiary rocks. The last division of the Elasmobranchii - viz., that of the Holocephali - is poorly represented in past time by Mesozoic and Kainozoic genera such as Ischiodus, Elasmodus, Ganodus, and Edaphodus.

III. The order of the Dipnoi, until of late years, was not known to be represented in past time at all. By the discovery, however, of the Queensland Ceratodi, it is now known that the Triassic and Jurassic teeth (fig. 274), upon which the genus Ceratodus was originally founded by Agassiz, are truly referable to Dipnoous fishes, mostly closely allied to the living forms. No other form of the Sirenoid division of the Dipnoi is known save Ceratodus; but the Ctenodipterine division of the order is well represented in past time by the Dipterus of the Devonian, the Ctenodus of the Devonian and Carboniferous, and some other less important genera.

IV. The Bony or Teleostean fishes do not make their appearance sooner than the Cretaceous period - that is, towards the close of the Mesozoic epoch. From this time on, however, Bony fishes with cycloid or ctenoid scales are the chief representatives of the whole class, and the order appears to have attained its maximum in our present seas.