182 ZOOLOGY SECT. 



known is a species of Ray, which extends to a depth of over 600 

 fathoms. 



None of the Elasmobranchs are of very small size, and comprised 

 among them are the largest of living Fishes : the harmless Basking 

 Sharks (Selache) sometimes attain a length of 35 feet or more, 

 the formidable Great Blue Shark (Garcharodon) sometimes 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, certain of which, if their general dimensions 

 were in proportion to the size of their teeth, must have reached a 

 length of as much as 60 feet. 



The earliest fossil remains of Elasmobranch Fishes that have 

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

 Throughout the Palseozoic 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 Paleozoic 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 Palseozoic 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 calci- 

 fication of the spinal column from the Palaeozoic forms onwards, 

 the Protoselachii alone among existing 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. Mitsukurina with long rostrum, occurring off 

 the coasts of Japan and Australia, dates from the Cretaceous 

 (Scapanorhynchus) . 



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 

 Chimcera, Callorhynchus, and Harriotta. Even taking in fossil 

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



