50 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



uous vertebrae. This condition I am not at present able to explain. 



If now the hypocentrum should vanish and its place be taken by 

 the enlarged pleurocentrum, as is believed to be the case among 

 the Amniota, the bases of the lower arches, if present, would 

 primarily fall between the vertebral centra. And it is between 

 the vertebrae, or close to their articulation, that we find the attach- 

 ments of the lower arches, or chevron-bones, of most of the higher 

 vertebrates. The departures from the rule may be easily explained 

 as secondary modifications. 



The upper arches, like the lower, are developed in the transverse 

 septa. This being the case, the connection of the arches with the 

 pleurocentra must be secondary. Hasse, in his " Beitrage zur Allge- 

 meinen Stammesgeschichte der Wirbelthiere," appears to have op- 

 posed this view, regarding, as other writers have done, the arches as 

 developing in direct union with the centra. On the other hand, Fro- 

 riep (27) holds that the body of the vertebra has an origin independ- 

 ent of the arches. The arch is the fundamental and earliest struc- 

 ture, the centrum a secondary one. He rejects the hypothesis that 

 the vertebral body of the higher vertebrates proceeds from the union 

 of the basal portions of the arches. According to Froriep, the prim- 

 itive membranous arches grow around the notochord and unite below 

 the latter to form his " hypochordale Spange." The latter is evi- 

 dently the representative of the hypocentrum. The upper arch at 

 length chondrifies and unites with the centrum next behind. The 

 latter is the pleurocentrum. The shifting of the arch backward to 

 become attached to the pleurocentrum reminds us of the movement 

 of the arches of Amia to join partly the pleurocentrum next behind. 

 Hoffman (40) tells us that in the tortoises the ribs and the upper 

 arches, during the early stages of development, rest intervertebrally 

 on the cartilaginous tube which surrounds the notochord. According 

 to Gegenbaur, there is, in Lacertilia, a stage in which the upper arch 

 is placed intervertebrally. (29, p. 44). The ribs of the amphibia 

 being developed in the transverse septa, must have their connection 

 primitively with the hypocentra, rather than with the pleurocentra. 

 It has been shown by Cope (14, p. 518) that in some of the Pelyco- 

 sauria (Theriodontia) the head of the rib is attached to the intercen- 

 trum ; and Dr. Baur has pointed out (6, 8) that this is true also in the 

 case of the cervical vertebrae of the very primitive reptile Spheno- 

 don, of those of the Crocodilia, and of those of some of the Dino- 

 sauria. When now the hypocentrum becomes rudimentary or disap- 

 pears, the head of the rib will, at first at least, be attached between 

 the pleurocentra, now become the centra. Such is its attachment in 



