1893.] TEBTEBBAIi AND LlilB-SKELETON OF THE AMPHIBIA. 273 



facts of morphology of the occipital and anterior-vertebral regions 

 of the vertebrate body raise the question whether in the so-called 

 " atlas " of Amphibians we may not be dealing with the variously 

 modified homologues of at least a couple of vertebrae, the formative 

 blastema of which has in the Amniota become merged in the 

 occiput. 



Von Ihering has adduced ^ reason for believing that the nume- 

 rical reduction of the vertebrae of Pipa to 8 is due to " escalation " 

 of the second vertebra, as normally enumerated in other forms ; 

 and the facts recorded by Portis^ for certain of the Tertiary 

 Anura go far towards proving that the near ancestors of the 



12 



So-called atlas of Siredon and Rana. 



Fig. 7- The so-called atlas of the Mexican Axolotl, Siredon pisa'formis, from 

 the side. Fig. 8. The same in JRana macrodon, exceptional. Fig. 9. 

 The same in Rana cateshiana, exceptional. Figs. 10 & 11. The same 

 in Rana esculenta, showing degrees in the occlusion of the foramen of 

 exit of the trans-atlantal nerve. Fig. 12. The same in R. esculenta, 

 normal. Figs. 7, 8, 10-12 enlarged. 



References as for figs. 3 to 6. 



living Anura possessed more than nine free vertebrae. While 

 in one of the two transverse-process bearing Progs' atlases herein 

 described the nerve foramina (w, fig. 10) were, like those of Prof. 

 Stewart's Eana cateshiami {n, fig. 9), widely open, in the other 

 (fig. 11) they were, as in his i?. macrodon (fig. 8), minute and re- 

 duced almost to occlusion. The differences between the first and 

 last named and the atlas of the normal or dominant type met with 

 among Anura (ex. fig. 12) are precisely those which would be 

 realized during the stages in suppression by fusion of a vertebral 

 segment ; and, in consideration more especially of the greater 



^ Morpholog. Jahrb. Bd. vi. p. 297 (1880) ; cf., however, Adolphi, loc. cit. 

 p. 315. 

 2 Atti Aocad. del. Sci. Torino, vol. xx. p. 1173 (1885). 



