ELASMOBRANCHII. 335 



the Sharks, there is only a single branchial aperture or gill-slit 

 externally, and this is protected by a rudimentary operculum 

 and branchiostegal rays. 



The order Elasmobranchii is divided into the two sub-orders 

 of the Holocephali and Plagiostomi. The former comprises the 

 living Chimczrce, characterised by the mouth being terminal, 

 and there being only a single gill-slit The latter comprises 

 the living Port Jackson Shark, the true Sharks and Dog-fishes, 

 and the Rays, characterised by having the mouth transverse 

 and placed on the under surface of the head, whilst there are 

 several apertures to the gills. 



As regards their general distribution in time, the Elasmo- 

 branchii are nearly as ancient as the Ganoids. At the top of 

 the Upper Ludlow Eocks, or at the close of the Upper Silurian 

 epoch, there have been discovered the remains of undoubted 

 Plagiostomous fishes, most nearly allied to the existing Port 

 Jackson Shark (Cestracion Philippi). These remains consist 

 chiefly of defensive spines, which formed the first rays in the 

 dorsal fins, and upon these the genus Onchus (fig. 295) has been 

 founded. Besides these there have been found portions of skin 

 or " shagreen," with little placoid tubercles, like the skin of a 

 living shark. These have been referred to the genera Sphago- 

 dus and Thelodus. They are the earliest known remains of 

 Plagiostomous fishes, and with the exception of the few re- 

 mains 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 Murchi- 

 son, the author of ' Siluria.' 



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. Subjoined is an illustration of the " ichthyo- 

 dorulites " and teeth of some of the Palaeozoic Cestraphori. 



The true Sharks are represented in the Mesozoic deposits 

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

 Cretaceous and Tertiary. The teeth of Odontaspis, Galeocerdo, 

 and Carcharodon, are good examples from the Eocene of the 

 Isle of Sheppey. The true Rays are older than the true 

 Sharks, the Carboniferous fossil Pleuracanthus being probably 

 the spine of a Ray (fig. 296). Numerous remains of Rays, 

 chiefly in the form of the pavement-like teeth, are known, 



