loo PRIMARY FACTORS OF ORGANIC EVOLUTION. 



zations at one side of the main line of descent of the 

 fishes. We look then for the ancestral type of the true 

 fishes among the Elasmobranchii, and of these the 

 Ichthyotomi display the greatest resemblances to the 

 Teleostomata in all respects. ^ 



Too little is known of the history of the subclasses, 

 excepting the Teleostomata, for us to be able to say 

 much of the direction of the descent of their contained 

 orders. On the sharks much light is shed by the dis- - 

 covery of characters of the genus Cladodus Agass., in 

 which the support of the paired fins consists of a meta- 

 pterygium, which is enclosed in a lateral fold of the 

 body wall, and which gives rise to simple external 

 basilar rods only. Of the Teleostomata a much clearer 

 history is accessible. It has four primary divisions or 

 tribes which differ solely in the structure of the sup- 

 ports of the fins. In the first division, the Crossopte- 

 rygia, the anterior limbs have numerous basilar bones 

 which are supported on a peduncle of axial bones. 

 The posterior limbs are similar. In the second divi- 

 sion, or Podopterygia (the sturgeons, etc.), the pos- 

 terior limbs remain the same, while the anterior limbs 

 have undergone a great abbreviation in the loss of the 

 axial bones and the reduction of the number and length 

 of the basilar bones. In the third group, or Actino- 

 pterygia, both limbs have undergone reduction, the 

 basilar bones in the posterior fin being almost all atro- 

 phied, while those of the fore limb are much reduced 

 in number. In the fourth superorder, the Rhipido- 

 pterygia, the axial supports of the median fins are 

 greatly reduced in number, presenting a marked con- 

 trast to the other superorders ; while the axial elements 



1 See Proceedings American Philosophical Society^ 1884, p. 572, on the p;enus 

 Didymodus, 



