663 



greatly enlarged, the pleuroceutra completely reduced; as in the Labyrinth- 

 odontia in which the bodies of the vertebrae are only formed by the inter- 

 ceutra. In the young Labyrinthodonts however the pleurocentra are still 

 present, but are reduced by the extension of the intercentra. The 

 condition of Archegosaurus has been called by Cope rhachitomous ; 

 that of Cricotus and Diplovertebron embolomerous. From 

 the rhachitomous condition that of the Amniota was developed. The 

 pleurocentra united to form the centrum of the vertebra the inter- 

 centra became more or less reduced. In the Sphenodontidae, the 

 Geckonidae, Uroplatidae still we find the small intercentra between 

 all the vertebrae. In the Lacertilia and Ichthyosauria they are con- 

 fined to the anterior cervical region. In the Magalosauria, Iguan- 

 odontia and Birds for instance, the first intercentrum forms the lower 

 piece of the atlas ring and the second intercentrum is united with 

 the centrum of the atlas (odontoid process) and centre of the axis 

 into one mass. In all mammals the first intercentrum always remains, 

 forming the lower piece of the atlas ring, and in some mammals they 

 are even present in the dorso-lumbo-sacral region, for instance in the 

 Insectivora (Talpa, Erinaceus, Myogale) and] in Atherura among the 

 Kodents (Parsons). 



We are able to explain the morphology of the vertebral column 

 of the Amniota in the easiest way by the condition seen in the 

 Stegocephali. A very similar condition to that of the Stegocephali 

 we find among the Cycloganoidei (Amiadae) as has been shown 

 very plainly lately by Hay in his paper on the structure and develop- 

 meut of the vertebral column of Amia. The conditions seen in the 

 Cycloganoidei is a nearly complete parallelism to those in the Stego- 

 cephali. In the Crossopterygii we find unossified and well ossified 

 vertebrae, it is very probable that some of the Crossopterygians showed 

 a condition like Archegosaurus and Caturus. 



The ribs. 



The question of the morphology of ribs is of the greatest import- 

 ance for the phylogeny of the Vertebrata ; especially in regard to the 

 origin of the Batrachia from fishes. As it is well known in all 

 Batrachia and Amniota the ribs are placed between the epaxonic and 

 hypaxonic muscles, they are pleurapophyses or pleural arches; in all 

 the Teleostomi, the Dipnoi included, the ribs are placed between the 

 hypaxonic muscles and the somatopleura. They are therefore haemapo- 

 physes. 



In the Elasmobranchii we have pleurapophyses which are attached 



