xliv. PRESIDENT'S ADbRBSS. 



particular conditions of the life of a fish, besides those which 

 mark its lower grade of development. With the exception of the 

 Amp/iinxus, the Lancelet, and the Cyclostomi t Lampreys, the 

 dermal covering of all existing fishes is strengthened by carti- 

 laginous or by osseous scales, enclosed within the substance of the 

 true skin. These scales are of various shapes and sizes, and 

 in some instances merely stud the skin at intervals ; in others 

 they are in close contact at the edges or cover each other 

 in an imbricated manner, or are developed into large plates, and 

 firmly united into a sort of a cuirass, as is the case with many of 

 the Palaeozoic fish, especially those of the Old Red Sandstone. 

 Nearly all the osseous fish of the present day have cartilaginous 

 scales. The most simple instance of bony-scales is seen in the 

 shark tribe, where the skin is beset with small tubercles, giving it 

 a roughness, which is known as shagreen. The most perfect 

 instance of the dermo-skeleton of a fish is where the bony- 

 scales meet at their edges, and are covered externally with a 

 lustrous surface. 



In the Palaeozoic age many of the predatory fish had 

 palatal as well as jaw-teeth, for crushing the bony armour 

 with which their prey was invested. The Teleostei, which 

 form the majority of the fish of the present day, are the 

 successors of the Ganoids, an Order to which the majority 

 of the fossil remains of the Palaeozoic and Mesozoic Ages 

 belong. They made their first appearance not earlier than 

 the Chalk. Instead of bony-plates their bodies are covered 

 with thin elastic scales and a well ossified endo-skeleton. After 

 this period the fish with palatal teeth began to decline. The 

 shark-tribe, which made their first appearance in the Devonian 

 Age, and culminated to 25 known genera, have now only 

 one living representative, Cestracion philippi, the Port Jackson 

 Shark. 



The embryo of vertebrates passes through several changes 

 before arriving at maturity. This can be traced in certain fossil 

 groups which had preceded them. Their ancestral history can be 

 traced by this metamorphic development. These changes are of 





