662 PISCES. 



has Disappeared from the ocean subsequently to its nrst introduction ; nor is there any that seems to have under- 

 gone any diminution ; so that the Ctenoid and Cycloid Orders may be said to be presenting their highest develop- 

 ment at the present time. 



When we survey the Geological distribution of the other two Orders, however, we see a most extraordinary 

 contrast. Although they now form so small a part of the inhabitants of our seas, we look back to a time when 

 they were the sole Vcrtebrated tenants of the globe; and we see that in the period anterior to that of the 

 predominance of the great extinct aquatic Reptiles, whose remains abound in the Lias and Oolite formations, 

 certain tribes of each Order had attained a very high degree of importance. Of several families, moreover, which 

 existed in the earlier periods of the history of the globe, some of them even having been the most numerous and 

 important tribes in the whole class as then existing, not a single representative now remains. Of these, the most 

 remarkable among the Ganoid fishes is the family of Lepidoids ; which was characterized by the possession of 

 numerous rows of brush-like teeth, and by the covering of flat rhomboidal scales arranged parallel with the body. 

 Remains of this family present themselves in nearly the oldest fossiliferous strata; it first began to abound, how- 

 ever, in the Carboniferous period; attained its fullest development in the period of the Triassic formation; 

 slightly diminished during the Jurassic period ; underwent a still greater diminution during the deposition of the 

 Chalk; and disappeared completely in the Tertiary epoch. Scarcely less remarkable is the history of the family 

 of Sauroid fish, so named from the numerous points of resemblance to Saurian reptiles, which occur in their 

 internal structure. This family seems to have commenced somewhat later than the preceding, but to have 

 attained its fullest development at about the same part of the series. Its existence, however, has been continued 

 downwards to the present time ; though it is now represented by only two genera, both of them restricted to fresh 

 water, — viz. the Tolypterus, an inhabitant of the rivers of Western Africa, and the Lepidosteus, a tenant of the 

 rivers and lakes of North America. 



In the Placoid Order, the family of Cestracionts corresponds very closely in its history with the Sauroid and 

 Lepidoid Ganoideans. This family bears a general resemblance to the Sharks in the form of the body and in 

 internal structure ; but the teeth, instead of being sharp and lancet-shaped, are flat and pavement-like, adapted 

 for crushing instead of for cutting. Remains of Cestraciont fish are among the earliest that present themselves 

 in the Palaeozoic rocks ; the family increased in importance through the Carboniferous series, and attained its 

 greatest development in the Triassic ; after which it progressively diminished, and is now represented by only a 

 single species, the Cestracion Philippi, or Port Jackson Shark. 



The families of Ganoid and Placoid fishes, which are now most numerous, may, for the most part, be traced 

 backwards to the remoter epochs. Thus the Sturgeons and the Rays have existed, nearly in the same proportion 

 as at present, from the beginning of the Lias formation ; the Chimoeroid fish date from the commencement of the 

 Secondary period ; whilst the Sqiialoids, or true Sharks, make their first appearance in the seas of the Carboni- 

 ferous epoch, and have been gradually increasing in importance down to the present time. It is very instructive 

 to compare the present predominance of these sharp-toothed Sharks, with the former high development of the 

 Cestracionts or blunt-toothed Sharks ; and to note how closely the gradual increase of the one tribe corresponds 

 with the decrease of the other. When we view these facts in connection with the general condition of the class 

 at each epoch, we find the explanation of it perfectly easy; for the period of highest development of the Cestra- 

 ciont family was that at which nearly all other existing fish were of the Ganoid order; that is, were covered 

 with an armature of bony or enamelled scales or plates, quite impenetrable to any simple cutting instrument, and 

 requiring powerful crushing teeth to make any impression upon them ; whilst, on the other hand, it is only since 

 the introduction of the Ctenoid and Cycloid fish, whose thin horny scales present no such impediment, that we 

 find the family of sharp-toothed Sharks, to which they furnish appropriate food, rising into importance. 



Omitting from the Ganoid Order the Siluroid family, which more properly belongs to the Cycloids, and omitting 

 from the Placoid Order the Cyclostome fish, which cannot be appropriately ranged under any one of the primary 

 divisions of Professor Agassiz, it may be stated as a general fact, that all the existing Ganoid and Placoid fishes 

 are representatives of families, whose first appearance dates back at least as far back as the commencement of 

 the Tertiary epoch, most of them being of much older date. The principal development of these Orders shows 

 itself in the Palaeozoic and Secondary periods, that is, during the formation of all rocks older than the Chalk; 

 and the class being then entirely destitute of Ctenoid and Cycloid fishes, must have possessed, as a whole, a very 

 different aspect from that which it now presents. 



Another general fact of much scientific interest is brought into view by the study of the fishes of the older 

 Geological formations. In all the Ctenoid and Cycloid fishes, the caudal fin is equally expanded above and below, 

 and commences from the end of the vertebral column, which does not pass into it. This form of tail is called by 

 Professor Agassiz, the homocercal tail. But in the Sharks, Sturgeons, and Lepidosteus of the present time, we find 

 the caudal fin composed of two unequal branches, of which the upper one is supported upon a prolongation of 

 the vertebral column, whilst the lower and shorter is given off from its under side. This form of tail is called 

 the heterocereal. All the earlier Placoid and Ganoid fish seem to have possessed this latter form of tail ; the 

 homocercal confirmation not manifesting itself until after the commencement of the Secondary period. Both 

 varieties present themselves in the Sauroid and Lepidoid fish ; the heterocereal in the older, and the homocercal 

 in the more recent. It is a curious fact that aU fishes have the heterocereal character of tail at an early period 

 of their embryonic development ; so that this may be considered as the more general form, which gives place, in 

 certain cases, to one more specially adapted to the conditions of their existence. 



Although the classification of Professor Agassiz has thus been of the greatest service to the Geologist, and was 

 a great advance upon that of Cuvier as regards its adaptation to the reception of the extinct forms of the class, 

 it has the faults to which all classifications that are based on single characters are liable; dissimilar tribes being. 



