xiii PHYLUM CHORDATA 173 



Xone of the Elasmobranchs are of very small size, and com- 

 prised among them are the largest of living Fishes. The harm- 

 less Basking Sharks (Selache) sometimes attain a length of 35 feet 

 or more, the formidable Great Blue Shark (Carclmrodon) some- 

 times reaches 40 feet, and some of the Rays also attain colossal 

 dimensions. In this respect, however, recent Sharks and Rays are 

 far behind some of the fossil forms, some of which, if their general 

 dimensions were in proportion to the size of their teeth, must 

 have attained a length of as much as sixty feet. 



The earliest fossil remains of Elasmobranch Fishes that have 

 been found occur in rocks belonging to the Upper Silurian period. 

 Throughout the Palaeozoic Epoch the Elasmobranchs constituted a 

 very important section of the fauna a large proportion of the fish- 

 remains that have been found in palaeozoic formations being the 

 remains of Elasmobranchs, mainly in the form of spines and 

 teeth. Most of the palaeozoic Elasmobranchs were characterised 

 by a great development of the exoskeleton. The teeth differ 

 from those of existing forms in being provided with broad bases 

 by means of which they articulated together, and in various 

 groups there is a union of the teeth by the coalescence of their 

 bases so as to form broad crushing plates. A similar union is not 

 uncommon between the parts of the general exoskeleton, a good 

 many palaeozoic Sharks having been encased in an armour of solid 

 plates formed by such a coalescence. In the endoskeleton there 

 is to be observed among the fossil Elasmobranchs a gradual 

 advance in the degree of calcification of the spinal column from 

 the palaeozoic forms onwards, the Protoselachii alone among exist- 

 ing forms representing in this respect the condition which seems 

 to have prevailed in the most ancient members of the class. 



The group (Cestracionts) now represented by two or three 

 species of Port Jackson Sharks seems to have been very abundant 

 in palaeozoic times. 



The extinct Pleuracanthea, together with Cladoselachus, which, 

 as briefly stated in the sketch of the classification, differ from the 



$/ 



other known members of the class in the structure of the fins and 

 other points, range from the Devonian to the Permian, and are 

 perhaps also represented in the Trias. 



Sub-Class II. Holocephali. 



The existing representatives of the Holocephali are included 

 under the single family Chimceridce, containing three genera- 

 Chimcem, Callorhynchus, and Harriotta. Even taking in fossil 

 forms, the group is a very small one ; it agrees in many funda- 

 mental characteristics with the Elasmobranchii, but presents so 

 many important differences that it cannot well be included in 

 that sub-class. Of the recent genera, Chimaera, the so-called 



