272 PHYLOGENY OF TUB CHORD AT A. 



placed mouth. Owing to the development of food-yolk in the Elas- 

 mobranch ovum the early stages of development are to some extent 

 abbreviated, and almost all trace of a stage with a suctorial mouth 

 has become lost. 



We next come to an hypothetical group which we may call the 

 Proto-ganoidei. Bridge, in his Memoir on Polyodon\ which contains 

 some very interesting speculations on the affinities of the Ganoids, 

 has called this group the Pneumatocoela, from the fact that we find 

 for the first time a full development of the air-bladder, though it is 

 possible that a rudiment of this organ, in the form of a pouch opening 

 on the dorsal side of the stomachic extremity of the oesophagus, was 

 present in the earlier type. 



Existing Ganoids are descendants of the Proto-ganoidei. Some of 

 them at all events retain in larval life the suctorial mouth of the 

 Proto-vertebrata ; and the mode of formation of their germinal layers, 

 resembling as it does that in the Lamprey and the Amphibia, probably 

 indicates that they are not descended from forms with a large food- 

 yolk like that of Elasmobranchii, and that the latter group is there- 

 fore 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 Stu- 

 riones) 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. 98). 



The Teleostei, which have in common a meroblastic segmenta- 

 tion, 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 segmen- 

 tation has been preserved. It is quite possible 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 descendants 

 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-penta- 

 dactyloidei, 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. Tlie 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. 



' /'/(//. Tnin^. 1H7S. Part ii. 



