300 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



form of teeth (Thelodus) and fin-spines or ichthyodorulites (On- 

 chus, Ctenacanthus). The order is but very scantily represented in 

 the Devonian deposits, but in those of the succeeding Carboniferous 

 age (sub- Carboniferous limestone) it acquires a profuse develop- 

 ment, the pavement-teeth of forms generally considered to be allied 

 to the modern " Port- Jackson " shark (Cestracion) being particu- 

 larly abundant Helodus, Orodus, Chomatodus, Petalodus, Cochlio- 

 dus, Psammodus. The relationships of these genera are, however, 

 still very obscure, and not impossibly they constitute a group apart 

 by themselves Psammodontes. Less doubtful representatives 

 (Strophodus, Acrodus, Hybodus) of the type of cestraciont fishes 

 appear in the early and middle Mesozoic periods, Triassic and 

 Jurassic, and in the chalk the teeth of Ptychodus are still very 

 abundant. But in the latter period the more modern type of 

 sabre-toothed sharks and dog-fishes, largely represented by existing 

 genera, Notidanus, Scyllium, Lamna, Carcharias, Hemipristis, Gale- 

 ocerdo (Corax, Otodus extinct), appear to have gained the ascend- 

 ency, which they retained throughout the subsequent Tertiary pe- 

 riods. The most important of the later genera, and especially 

 distinctive of the Miocene deposits, is Carcharodon. The earliest 

 unequivocal traces of rays occur in the deposits of Jurassic age, 

 although not impossibly some of the hypothetically placed forms 

 of the later Paleozoic periods may belong to this group, or, at any 

 rate, effect a union between it and the sharks. Of this nature 

 appear to be the Jurassic Thaumas and Squaloraja, both of them 

 nearly allied to the modern (and Cretaceous) Squatina. The true- 

 and sting-rays are abundantly represented, particularly by frag- 

 ments of their dental armatures, throughout the Tertiary deposits r 

 the modern genera (Trygon, Myliobatis, JEtobatis, Zygobatis, Raja) 

 predominating. The saw-fish (Pristis) and torpedo-ray date from 

 the Eocene, and not impossibly from the Cretaceous period. 



Appearing almost simultaneously with the selachians, but at- 

 taining a much earlier considerable development, are the ganoids, 

 a class of fishes which in the earlier geological periods exhibit a 

 remarkable diversity of structure, but at the present day are com- 

 prised within a very limited number of genera, whose members, 

 with the exception of the partially marine sturgeon (Accipenser), 

 are all inhabitants of fresh water. Three or more distinct types, 

 based upon characters drawn from the structure of the dermal 



