ON THE ANCESTRAL FORM OF THE CHORDATA. 329 



and the Amphibia, probably indicates that they are not de- 

 scended from forms with a large food-yolk like that of Elasmo- 

 branchii, and that the latter group is therefore a lateral offshoot 

 from the main line of descent. 



Of the two groups into which the Ganoidei may be divided 

 it is clear that certain members of the one (Teleostoidei), viz. 

 Lepidosteus and Amia, shew approximations to the Teleostei, 

 which no doubt originated from the Ganoids ; while the other 

 (Selachoidei or Sturiones) is more nearly related to the Dipnoi. 

 Polypterus has also marked affinities in this direction, e.g. the 

 external gills of the larva (vide p. 118). 



The Teleostei, which have in common a meroblastic segmen- 

 tation, had probably a Ganoid ancestor, the ova of which were 

 provided with a large amount of food-yolk. In most existing 

 Teleostei, the ovum has become again reduced in size, but the 

 meroblastic segmentation has been preserved. It is quite possi- 

 ble that Amia may also be a descendant of the Ganoid ancestor 

 of the Teleostei ; but Lepidosteus, as shewn by its complete 

 segmentation, is clearly not so. 



The Dipnoi as well as all the higher Vertebrata are descen- 

 dants of the Proto-ganoidei. 



The character of the limbs of higher Vertebrata indicates 

 that there was an ancestral group, which may be called the 

 Proto-pentadactyloidei, in which the pentadactyle limb became 

 established ; and that to this group the common ancestor of the 

 Amphibia and Amniota belonged. 



It is possible that the Plesiosauri and Ichthyosauri of 

 Mesozoic times may have been more nearly related to this 

 group than either to the Amniota or the Amphibia. The 

 Proto-pentadactyloidei were probably much more closely related 

 to the Amphibia than to the Amniota. They certainly must 

 have been capable of living in water as well as on land, and had 

 of course persistent branchial clefts. It is also fairly certain 

 that they were not provided with large-yolked ova, otherwise 

 the mode of formation of the layers in Amphibia could not be 

 easily explained. 



The Mammalia and Sauropsida are probably independent 

 offshoots from a common stem which may be called the Proto- 

 amniota. 



