CLASSIFICATION 



117 



at.-— 



their phylogenetic tree have been pruned off by natural selection, 

 that the branches which survive to the present clay can be easily 

 classified into well-defined groups. But when we come to deal with 

 the extinct forms, we are at once met Avith the familiar difficulties 

 in the construction of a phylogenetic system : the discovery of 

 intermediate forms ; apparent primitive simplicity due to degenera- 

 tion ■ apparent close relationship due 

 to convergence. 



In the eighteenth century, Artedi, 

 a friend of Linnaeus, founded the 

 modern classification of fish by dividing 

 them into three groups : the Chon- 

 dropterygii, the Malacopterygii, and 

 the Acanthopterygii. A great advance 

 was made in the first half of the nine- 

 teenth century in our knowledge of 

 the structure and classification of fish 

 by the publication of Cuvier and Valen- 

 cienne's Histoire Naturelle des Poissom 

 [95]. These authors separated all 

 the ' bony fish ' as one group from 

 the Chondropterygii. But among the 

 latter were still included the Cyclo- 

 stomes and the Sturgeons, with the 

 Elasmobranchs. A third epoch was 

 marked by the appearance of the 

 work of L. Agassiz on fossil fish [4]. 

 Belying chiefly on the characters of 

 the exoskeleton, he divided the true 

 Bisces into Blacoidei, Ganoidei, 

 Cycloidei, and Ctenoidei. The skin 

 of the first division is provided with 

 ' placoid ' denticles ; that of the 

 second, with thick shiny bony scales ; 

 the third, with rounded, thin, over- 

 lapping, concentrically lined scales ; 

 the fourth, with similar scales bear- 

 ing spiny processes behind. Although * J ***& fitoffigjgffi 



Agassiz greatly forwarded the Study inner free edge of valve ; st, stomach. 



both of living and of extinct fish, 



yet his classification in these four orders is not natural. The 

 order Ganoidei included several unrelated but convergent forms, 

 such as Polypterus, Aripemer, Ostracion, and Lepidosiren ; and 

 the separation of the Cycloidei from the Ctenoidei was very 

 artificial. Johannes Miiller [307] united these two orders in the 

 sub-class Teleostei ; and purged the Ganoidei of the Dipnoi and of 



Fig. 78. 



Intestine of Zygaena malleus, opened 

 up to show the spiral valve. (After 



