258 Dr. H. Gadow. On the Evolution of the [June 20, 



type, e.g., trunk of Euchirosaurus, Actinodon, Trinero- 

 rhachis.) 



2. The three pairs of basidorsalia, interdorsalia, and basi- 

 ventralia co-ossify. (Trunk vertebrae of most recent 

 Amphibia, caudal vertebras (coccyx) of Anura.) 



III. Interventralia and basiventralia are reduced. The vertebrae 



consist of dorsal elements only, so-called " epichordal " 

 type. (Trunk of Pipa, Xenopus, Bombinator,) 



IV. The interdorsalia are reduced. The interventralia are enlarged 



and form the centrum. (GASTROCENTROUS type.) 

 IVA. The three remaining 1 components ossify separately and remain 

 separate ; the basiventralia are still large and carry most of 

 the neural arch. (All the vertebras of Eryopse, Criootus, the 

 atlas of the Amniota.) 



IVs. The centrum is much enlarged, forms the whole of the " body," 

 and carries the neural arch (neurocentral suture). The 

 basiventralia are much reduced and form the " intervertebral 

 disks " or menisci, attached to the cranial end of the 

 centrum ; when less reduced, they appear as the so-called 

 wedge bones, " intercentra," or chevrons. (Trunk and tail 

 of most Amniota.) 



IVc. The centrum and the neural arches alone constitute the ver- 

 tebra. The basiventrals are lost completely. (Trunk of many 

 Amniota, for instance, the thoracic and lumbar vertebrae of 

 lizards, caudals of many mammalia and birds.) 



The phylogeny of these modifications maybe expressed as follows : 



I. 1 



Pseudocentra. I. 2 /\ 



/ \ 

 frotocentra. II IV Gastrocentra. 



/ 



/ \A 



III \ 



\B 



s \c 



The ribs are homologous structures throughout the vertebrata. 

 They are lateral distal outgrowths of the original basiventralia, with 

 which they later on form joints, and they ossify independently. 



The " chevrons," the bony arms or arches which enclose the caudal 

 canal, are more ventral or median outgrowths of the same basiven- 

 tralia and are to be called haamapophyses. They are possibly serially 



