506 



MANUAL OF ZOOLOGY. 



living shark. These have been referred to the genus Sphagodus-; 

 They are the earliest known remains of Plagiostomous fishes, 

 and, with the exception of the few remains from the Lower 

 Ludlow rocks, they are the earliest known remains of fishes in 

 the stratified series. The discovery of these remains, at that 

 time the earliest known traces of Vertebrate life, is due to the 

 genius of Sir Roderick Murchison, the author of ' Siluria.' 



Fig. 275. i. Spine of Pletir acanthus (one of the Rays); 2. Gyracanthus ', 3. Ciena- 

 cant 'hits ; 4. Tooth of Petalodus ; 5. Psammodus; 6. Ctenoptychius. All from the 

 Carboniferous rocks. 



Most of the fossil Elasmobranchii belong to the division 

 Cestraphori of Owen, so called because they are provided with 

 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. 275) is an illustration of 

 the " ichthyodorulites " and teeth of some of the Palaeozoic 

 Cestraphori, 



The true Sharks are represented in the later Mesozoic 

 deposits (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, 

 Pleuracanthiis, being probably the spine of a Ray (fig. 275). 

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



