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t^e Placoids and Ganoids existed before ttie commencement 

 of the Cretaceous formations. The Ctenoids and Cycloids, 

 which contain 600 out of the 8000 known living species 

 af fishes, appear for the first time in the Cretaceous strata, 

 when all the preceding fossil genera of the two first orders 

 had become extinct, which are those only to which we shall 

 have to refer. Of the Placoidians, the most frequent 

 remains observed in the Coal formation belong to the Shark 

 family. The specimens which have come under my notice, 

 consist of the teeth and dorsal Spines, or Ichthyodorulites, 

 as they have been called, of the Hybodonts ; of these, fine 

 specimens of Gyracanthus formosus, and Pluracanthus 

 planus, have been found at Middleton and Adwalton; 

 Ctenacanthus brevis, and Orthacanthus cylindricus, in the 

 stone Coal at Adwalton; of the Cestracionts, only palatal 

 teeth have occurred, and these sparingly ; of a species of 

 Helodus and Ctenoptychius pectinatus, at Middleton ; also a 

 fine, and I believe unique, specimen of Ctenodus cristatus, 

 upon a piece of Coal from Tong, preserved in the Museum 

 of the Leeds Philosophical Society; teeth of Diplodus 

 gibbosus and Hybodus obtusus, at Middleton. M. Agassiz 

 also enumerates, Petalodus acuminatus, from Leeds, and 

 Carcharopsis prototypus, from Yorkshire, which I cannot 

 trace. As these animals are amongst the most universally- 

 diff'used and most voracious of the modern fishes, so there 

 is no period with which Geology is familiar, in which either 

 true Sharks or analogues equivalent to them, did not exist. 

 Agassiz determined the existence of more than 150 extinct 

 species of fossil fishes allied to this family. The first and 

 oldest, the Cestracionts, beginning with the Transition strata, 

 appears in every subsequent formation, till the commence- 

 ment of the Tertiary, and has only one living representative, 

 the Cestracion Phillipi of Port Jackson. The Hybodonts, 

 beginning with the Muschel Kalk, and probably Coal 

 formation, prevails throughout the Oolitic series, and ceases 



