HARD WICKE 'S S CIENCE- G OSSIF. 



275 



The Devonian formation is built up of freshwater, 

 estuarine, and marine strata, each group characterised 

 by its peculiar forms of life. In the Old Red Sand- 

 stone of Scotland and Russia, freshwater species 

 predominate, while in the marine limestones of 

 Devonshire and the Eifel, Mollusca, Corals, and the 

 remains of genera of inshore-dwelling fish indicate a 

 shallower marine deposit. The greater part of the 

 American Devonian, on the contrary, was apparently 

 laid down in a deep sea, and thus a monster marine 

 fauna flourished, not so generally represented in 

 Europe ; but it is interesting to note the identity of a 

 few species occurring in localities where the beds are 

 of similar structure to those of contemporaneous age 

 in Europe. In both worlds the formation is alike 

 distinguished by the great preponderance of ganoid 

 over elasmobranchiate fishes. The conditions existing 

 during the formation of the Devonian rocks are well 

 illustrated at the present day by the freshwater lakes, 

 mighty rivers, and extended coast line of the African 

 and American continents, and it is a most suggestive 

 and significant fact that the genera of living ganoid 

 and dipnoid fishes most resembling the palaeozic forms 

 are now, with two exceptions, found on those con- 

 tinents alone. Taking the various orders of Professor 

 Huxley's comprehensive classification in succession, we 

 find that no traces of the first or lowest order, the 

 Pharyngobranchii, which contains only the "gullet 

 breathing " Lancelet, have been found in a fossil state. 

 This is easily accounted for, however, by the soft and 

 perishable structure of the species, of which no 

 remains could possibly be preserved m the finest sedi- 

 mentary strata, and therefore the non-representation 

 of this lowest form of ichthyic life in "the records 

 of the rocks" becomes less remarkable. Of the 

 cartilaginous Marsipobraiichii, comprising the hag 

 fishes and lampreys, the horny teeth alone would be 

 susceptible of preservation, and their absence has 

 been commented on as negativing the evidence of 

 progressive development among fishes, as it is obvious 

 the most simply constructed forms should appear first 

 on the scene of life, in order to give place to their 

 more highly organized descendants. In 1856, Pander, 

 in his magnificent work on the Silurian and Devonian 

 fishes of the Russian Baltic provinces, gave numerous 

 figures of what he supposed to be the teeth of small 

 sharks firom the Lower Silurian rocks ; but these so 

 termed conodonts have not been accepted as of true 

 ichthyic origin. Professor Owen * retains only three 

 species as possibly the teeth of fishes, and is of 

 opinion that the remainder might be either the 

 ornaments of crustaceans, " or the spines, or booklets, 

 or denticles of naked mollusks or annelides. " Great 

 numbers of these "cone teeth" have recently been 

 detected in carboniferous strata both in England and 

 America, and it is suggested that they may be the 

 teeth of cyclostomous fishes like the hags and lampreys, 



* Enc. Brit., vol. xvii., part i. 1839, art. " Palseontology." 



and thus be the representatives of the MarsipobrancJiii 

 of the ancient Silurian seas. They seem most to 

 resemble in shape and structure the teeth of the 

 Myxinoids, in which the dentition is peculiar, being 

 composed of one horny conical tooth situated in the 

 roof of the mouth, with two serrated dental plates on 

 the tongue. It has been objected that the teeth of 

 living cyclostomous fishes are horny or chitonous, 

 while the fossil cone teeth are calcareous ; but this 

 applies with equal force to the theory that they are 

 the teeth of mollusks, as the modern shell-fish have 

 siliceous teeth. The piscine derivation of the 

 conodonts is, however, still a debated question 

 requiring careful investigation, as it would antedate 

 the appearance of ichthyic life in geologic history ; 

 but if it cannot be asserted that they are the teeth of 

 fishes, neither as yet can it be positively proved that 

 they are not. The next order, the Elasinohranchii, 

 embraces the sharks, dog-fishes, rays, and Ckim<2roids. 

 The first of these families has enjoyed a long range 

 from the Upper Silurian epoch to the present day, and 

 one genus seems to have varied but slightly, the 

 Cestracion Phillippi, or Port Jackson shark of Australia, 

 being a descendant of the old time Cesfracionfes, a 

 once numerous family now verging towards extinction. 

 The Chimceroids appeared first in the Devonian, and 

 live on, but the Rays were not represented until the 

 Jurassic age. The Placodenns, as we have seen, 

 enjoyed but a transient existence, dying out at the 

 close of the Devonian, while the Tdeostei, or true bony 

 fishes, which so largely predominate at the present day, 

 did not appear on the scene of life until the formation 

 of the cretaceous rocks. Seven living genera alone 

 survive of the Gajioidei, which prevailed so numerously 

 in PalcEozoic times, and but one of these, the Sturgeon, 

 the least characteristic of the group, is found in 

 European waters. Two of the six remaining forms, 

 which are all dwellers in fresh water, occur in Africa, 

 and four inhabit the lakes and rivers of North America. 

 The preservation of the majority of living ganoids in 

 America is probably owing to the fact that some 

 portions of this ancient continent, truly the old world 

 of geologists, have never been submerged since their 

 upheaval from the first Silurian seas : thus some repre- 

 sentatives of this ancient race of fishes were able 

 to find a refuge in its bays and rivers, and the chain 

 of descent has been kept unbroken from the early ages 

 of the incalculably remote past. The large-spined, 

 shagreen-scaled Acanthodidce, which are considered 

 by Professor Huxley to link the Ganoids to the 

 Elasmobranchs, range only in the Devonian and car- 

 boniferous rocks. The ' ' thick-toothed " Pycnodonts 

 lived from the coal-measures to the Tertiaries, and 

 are now extinct, while the buckler-headed Cephalaspids, 

 like the Placodenns, existed only in Devonian times. 

 The Chojtdrosteidce, to which group the Sturgeons 

 belong, were certainly represented in the Jurassic seas, 

 and possibly by the gigantic MacropetalichtJiys in the 

 Devonian. Amia calva, the dog-fish of the American 



