PEDIGREES. 191 



realisation of this, inasmuch as a fossil species 

 from the Eocene of Monte Bolea possesses a 

 caudal fin, which may be said to have since gone 

 out of fashion. 



The present chapter may be summarised 

 briefly as follows : 



The fishes of the present day may be divided 

 into two great groups according to the -structure 

 of the skeleton of the head. In one group we 

 have the ancient sharks and rays, and the 

 modern bony fishes represented by the salmon, 

 perch, and cod-fish, for instance. In the other 

 we have the curious chimeras, which will be 

 described presently, and the lung-fishes, which 

 we have discussed from some aspects (pp. 25, 67). 



The shark-tribe and the modern bony-fishes 

 are bracketed together because the upper jaw is 

 but loosely attached to the skull a type of 

 skull known as the hyostylic whilst in the 

 chimeras and lung-fishes the upper jaw is indis- 

 tinguishably welded to form one piece with the 

 skull, and on this account forms a second type of 

 skull, the autostylic. 



The hyostylic group of fishes are divided into 

 two sub-classes the Elasmobranchii and the Teleo- 

 stomi. 



The sharks and rays constitute the Elasmo- 

 branchii a name given in allusion to the strap 

 or band-like bars that divide the gill-slits. They 

 are distinguished from the Teleostomi by the 

 fact that the body is covered with "placoid" 

 scales (p. 34), and that the gill openings are 

 numerous and exposed. 



The modern bony fishes form the Teleostomi, 



