24 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



Zittel render it perfectly clear that the fishes rarely develop complete 

 vertebral centra [pleurocentra], the order Halecomorphi being the 

 only example." On the contrary many genera belonging to a 

 mimber of families are mentioned by Prof. Zittel as possessing pleu- 

 centra and hypocentra. 



4. DISCUSSION OF THE VERTEBRAL COLUMN OF THE HIGHER 

 VERTEBRATA. 



But the Ganoid and Teleost fishes are not the only vertebrates 

 which manifest in their vertebral centra the possession of the ele- 

 ments which we have found so well developed in the vertebrae of the 

 Amioid fishes. Hermann von Meyer was the first to point out that 

 the vertebrae of the fossil Archegosaurus, among the Amphibians, 

 were each made up of a numher of distinc t pieces. This genus, and 

 others closely related to it, show indeed a remarkable resemblance in 

 the structure of their vertebrae to the fishes related to Amia, espec 

 iaily to Caturus. Others again, as Cricotus and Diplovertebron, have 

 the vertebrae of at least the tail constructed exactly like the vertebrae 

 of the middle portion of the tail of Amia. Some other fossil genera, 

 as Mastodonsaurus, usually have solidly ossified vertebrae, but Von 

 Meyer has shown that such vertebrae may, during the youth of the 

 animal, have the rhachitomous structure of Archegosaurus. Some spe- 

 cies, again, as those classified as Lepospondyli, had simple tubular 

 vertebrae ; but in closely related genera, such vertebrae consisted each 

 of a right and left half, the two meeting by a suture along the dorsal 

 and ventral sides of the notochord. Whether in such cases we have 

 two pleurocentra or two hypocentra, or the two elements coalesced, 

 it may yet be impossible to say. Cope holds the view that the verte- 

 brae of modern Amphibians consist only of hypocentra. This view 

 will be discussed further on. 



On the other hand it has been shown by Cope, Albrecht, Dollo 

 and Baur, that the vertebral centra of the reptiles, birds and ani- 

 jnals are really pleurocentra, while the hypocentra appear only 

 occasionally in reptiles, especially as " subvertebral wedge-bones," 

 and in all Amniota as the " body " of the atlas. 



The finding of the elements pleurocentra and hypocentra in the 

 vertebral column of so many of the osseous fishes, the amphibians, 

 and the Amniota may well lead us to suspect that future investigations 

 will reveal a still more general participation by them in the structure 

 of the vertebral column. Such investigations may further prove to 

 what extent these elements enter into the constitution of the primi- 

 tive vertebra from which all others have been derived. 



