48 ANNUAL REPORT OF 



are typically Arctic. None of them are small, and some of the 

 sharks are the largest of living fishes. All are carniverous, but so 

 diversified is their food that in different species it may range from 

 other fishes of no mean size to mollusks, crustaceans and other 

 invertebrates, or even to plankton. In their breeding habits the 

 sharks and dog-fishes present many interesting features. * * * 

 The majority of the sharks, dog-fishes and rays are viviparous, 

 that is, the young are born alive ; the rest * * * are ovipa- 

 rous, that is, the young are hatched out after the extrusion of 

 the eggs." 



Fossil remains of Elasmobranchs in the shape of detached hard 

 parts, such as teeth, fin-spines and dermal tubercles, are known 

 from a few Silurian localities, and are therefore amongst the 

 earliest undoubted indications of vertebrate life. Fragments of 

 this description become more numerous in the Devonian, and in 

 the uppermost horizons of the system are found magnificently 

 preserved skeletons, which exhibit in some instances even the 

 microscopic structure of muscular tissues. 1 



Pig. 5- 



Cladoselache fyleri Newb. Cleveland Shale (Upper Devonian) ; Ohio. Lat- 

 eral aspect, anterior dorsal fin-spine omitted. X V20. (From Dean.) 



The best known of these primitive sharks is Cladoselache (Fig. 

 5), from .the Cleveland shale of Ohio, an elongated, round- 

 bodied form attaining a length of about five feet, with two^ dorsal 

 fins and a very remarkable caudal extremity. The structure of 

 the paired fins is extremely interesting in that it enlightens us 

 as to the probable origin of vertebrate limbs from continuous 

 dermal folds on either side of the body, just as the dorsal, caudal 

 and anal fins are presumably derived from a continuous median 

 fold. The teeth of Cladoselache are in the form of sharply 



1 Dean, B., Preservation of Muscle-fibres in Sharks of the Cleveland Shale. 

 Amer. Geol., vol. xxx. (1902), pp. 273-278. 



