188 ZOOLOGY sect. 



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 similaj- 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 calcification of the spinal column from 

 the Palseozoic 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 — ■ 

 Ghimcera, 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, and is sometimes 

 included in that sub-class. Of the recent genera, Chimtera, the 

 so-called " King of the Herrings" (Fig. 847, A) is found on the 

 coasts of Europe, Japan, and Australia, the west coast of North 

 America, and at the Cape of Good Hope ; Callorhynchus {B) is 

 tolerably abundant in the South Temperate seas ; Harriotta is 

 a deep-sea form. 



External Characters. — The general form of the body is Shark- 

 like, but the large, compressed head and small mouth are strikingly 

 different from the depressed, shovel-shaped head and wide mouth 

 of most Selachians. The mouth is bounded by lip-like folds, two of 

 which {B, l.f., l.f.'), placed laterally and supported by labial carti- 

 lages, resemble the folds in which the premaxilljB and maxillaj of 

 many Bony Fishes are enclosed : a third fold, external to and 

 concentric with the mandible, is also supported by labial cartilages 



