PLACOID, CTENOID, AND CYCLOID FISHES. 71 



Palasozoic strata; they become more numerous in the Car- 

 boniferous series ; they are very numerous in the Lias and Chalk 

 formations ; but there they cease almost entirely, the strata of 

 the Tertiary series scarcely containing any of them, and the 

 Port- Jackson Shark being the only representative of this family 

 at the present day. Intermediate between these and the ordinary 

 Sharks was another family, to which the name of Hybodonts has 

 been given. The teeth of this division were stronger and 

 blunter than . those of the true Sharks, but were not so much 

 flattened as those of the Cestracionts ; and they seem to have 

 been adapted for cutting, tearing, and bruising substances of 

 considerable hardness. The Fishes of this family seem to have 

 made their first appearance in the later part of the Coal forma- 

 tions ; they were very abundant during the Oolitic period ; but 

 ceased entirely at the commencement of the Chalk deposit. 

 Lastly, the ordinary Sharks, constituting the Squaloid family, 

 which are distinguished by their sharp lancet-like teeth, have no 

 representatives among the Fossil Fishes of older date ; but their 

 remains are first found in the Chalk formations, and extend 

 through all the newer strata down to the present time. The 

 Rays, also, of the earlier periods had teeth more flattened than 

 those of later epochs ; and we find from their fossil remains, 

 that some of the forms, which are now regarded as exceptional 

 or aberrant, were formerly more abundant. Thus of the 

 Myliolatis or Eagle-Ray ( 584), of which five species are at 

 present known, fifteen fossil species have been discovered. 



590. The Ctenoid and Cycloid Fishes make their first 

 appearance in the Chalk formation ; when all the previously- 

 existing genera of the Ganoid and Placoid orders had become 

 extinct, and when the new ones that were brought into existence 

 were (as we have seen) far less numerous than before. There is, 

 then, a sort of 'boundary line at the base of the Cretaceous or 

 Chalk deposits, which divides the class of Fishes in a most 

 remarkable manner ; all those below that line, in the order of 

 the strata, or (in other words) all those which existed at a period 

 anterior to the deposition of the Chalk, having belonged to the 

 two first orders, those with enamelled scales ; whilst by far the 



