CHAPTER II 

 THE GANOIDS Continued 



LASSIFICATION of Ganoids. The subdivision of the 

 series of Ganoidei into orders offers great difficulty 

 from the fact of the varying relationships of the mem- 

 bers of the group and the fact that the great majority of the 

 species are known only from broken skeletons preserved in the 

 rocks. It is apparently easy to separate those with cartilaginous 

 skeletons from those with these bones more or less ossified. It 

 is also easy to separate those with bony scales or plates from 

 those having the scales cycloid. But the one type of skeleton 

 grades into the other, and there is a bony basis even to the 

 thinnest of scales found in this group. Among the multitude 

 of names and divisions proposed we may recognize six orders, 

 for which the names Lysopteri, Chondrostei, Selachostomi, 

 Pycnodonti, Lepidostei, and Halecomorphi are not inappropriate. 

 Each of these seems to represent a distinct offshoot from the 

 first primitive group. 



Order Lysopteri. In the most primitive order, called Lysop- 

 teri (\vo-6s, loose; Ttrepov, fin) by Cope, Heterocerci by Zittel 

 and Eastman, and the "ascending series of Chondrostei" by 

 Woodward, we find the nearest approach to the Chondropter- 

 ygians. In this order the arches of the vertebrae are more or 

 less ossified, the body is more or less short and deep, covered 

 with bony dermal plates. The opercular apparatus is well 

 developed, with numerous branchiostegals. Infraclavicles are 

 present, and the fins provided with fulcra. Dorsal and anal 

 fins are present, with rays more numerous than their supports; 

 ventral fin with basal supports which are imperfectly ossified; 

 caudal fin mostly heterocercal, the scales mostly rhombic in 

 form. All the members of this group are now extinct. 



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