43O MANUAL OF ZOOLOGY. 



the large fin-spines, which are known to geologists as " ichthyo- 

 dorulites." The two families of this division the Cestracionts 

 and Hybodonts are largely represented in past time, the 

 former chiefly in the Palaeozoic period, the latter chiefly in the 

 Mesozoic rocks. Above (fig. 183) is an illustration of the 

 " ichthyodorulites " and teeth of some of the Palaeozoic Cestra- 

 phori. 



The true Sharks are represented in the later Mesozoic de- 

 posits (e.g., by teeth of Notidanus in the Oolites) ; but they are 

 chiefly Tertiary. The teeth of Odontaspis, Galeocerdo, and 

 Carcharodon, are good examples from the Eocene. The true 

 Rays are older than the true Sharks, the Carboniferous fossil, 

 Pleuracanthus, being probably the spine of a Ray (fig. 183). 

 Numerous remains of Rays, chiefly in the form of the pavement- 

 like teeth, are known, both from the Secondary and Tertiary 

 rocks. The last division of the Elasmobranchii viz., that of 

 the Holocephali is poorly represented in past time by the 

 Mesozoic and Kainozoic Ischiodus, Elasmodus, Ganodus, and 

 Edaphodus. 



III. The Bony or Teleostean fishes do not make their ap- 

 pearance sooner than the Cretaceous period that is, towards 

 the close of the Mesozoic epoch. From this time on, how- 

 ever, Bony fishes with cycloid or ctenoid scales are the chief 

 representatives of the whole class, and the order appears to 

 have attained its maximum in our present seas. 



