EVOLUTION OF THE VERTEBRATA. 325 



the descent of their contained orders. On the sharks some lijrht 

 is shed by the discovery of tlie genus Chlamydoselachus Garruan,* 

 which is apparently nearly related to the Cladodonts of the Devo- 

 nian seas. This genus has more numerous branchial slits than all 

 but two of the genera of existing sharks, and it differs from all but 

 these two in having a more perfect articulation between the tooth- 

 bearing bones and the cranium. Of the Teleostomi a much clearer 

 history is accessible. It has three primary divisions or tribes which 

 differ solely in the structure of the supports of the fins. In the 

 first division, the Crossopterygia, the anterior limbs have numerous 

 basilar bones which are supported on a peduncle of axial bones. 

 The posterior limbs are similar. In the second division, or Chron- 

 drostei (the sturgeons, etc.), the posterior limb remains 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 Actiuopteri,f 

 both limbs have undergone reduction, the basilar bones in the pos- 

 terior fin being almost all atrophied, while those of the fore limb 

 are much reduced in number. 



The phylogeny of these tribes is not easy to make out at pres- 

 ent. The descent has been, no doubt, in the order named in time, 

 but the starting-point is yet uncertain. Thus the Chondrostei 

 appear later in time than either of the other tribes, a history 

 which probably only represents our ignorance. The characters of 

 the genus Crossopholis Cope, from the American Eocene, strongly 

 sufTfjest that the existing forms have descended from scaled an- 

 cestors. The Crossopterygian fore limb, with its arm-like axis, 

 tells of the origin of the first limbed vertebrates, the Batrachia, 

 whose skull-structure, however, only permits their derivation 

 from the Dipnoi or Ilolocephali. As the former subclass has the 

 Crossopterygian fin-structure, we can safely regard them as the 

 ancestors of the Batrachia, while the Crossopterygia are a side- 

 line from a similar type, probably the Ichthyotomi, because these 

 have a free suspensor of the lower jaw. But of the structure of 

 the fins of the Ichthyotomi unfortunately we know nothing. If 

 this position be true, then the successive derivation of the Chon- 

 drostei and the Teleostomi in one line is rendered probable. The 

 modification of structure has consisted in the contraction of the 



* "Proceedings American Assoc. Adv. Sci.," 1884. 



f Partly agrees with the Teleostei of Miillcr, but includes many of his Ganoidea. 



