154 ORDERS OF FISHES. 



first rays in tlie dorsal fins, and upon these the genus Onchus 

 (fig. 520) lias been founded. Besides these there have been 

 found portions of slvin or " sliagreen," with little placoid 

 tubercles, like the skin of a living shark. These have been 

 referred to the genera Sphagodus and Tlidodiis. 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 Eoderick Murchison, the author of ' Siluria.' 



Most of the fossil Elasmohrancliii belong to the division 

 Ccstrcqoliori of Owen, so called because they are provided 

 with the large fin-spines which are known to geologists as 

 " ichthyodorulites." The two families of this division — the 

 Cestracionts and Hybodonts — are largely represented in past 

 time, the former chiefly in the Palseozoic period, the latter 

 chiefly in the Mesozoic rocks. 



The true Sharks are represented in the earlier Mesozoic 

 deposits {e.g., by teeth of Notidanus in the Oolites) ; but they 

 are chiefly Cretaceous and Tertiary. The teeth of Odontasjjis, 

 Galcocerdo, and Carcharodon, are good examples from the 

 Eocene of the Isle of Sheppey. The true Eays are older 

 than the true Sharks, occurring as early as the Carboniferous. 

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

 ment-like teeth, are known, both from the Secondary and 

 Tertiary rocks. The last division of the £lasmohra7icJiii — 

 viz., that of the Holoccjiludi — is poorly represented in past 

 time by the Ilhyncliodus of the Devonian, and by the 

 Mesozoic and Kainozoic Ischiodus, Masmodus, Ganodus, and 

 Edcqjihodus. 



In the following a more detailed account is given of the 

 characters of the various groups of the Elasmohrancliii with 

 the leading characters and more important fossil forms of 

 each : — 



SuB-OEDER A. HoLOCEPHALi. — This sub- Order includes 

 certain curious fishes, of which the only living forms are the 

 Chimmridce. The notochord is persistent, but the neural 

 arches and transverse processes are cartilaginous. The jaws 



