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Show 1893.] VERTEBRAL AND LIMB-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 w e m a y not be dealing wdth 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 x reason for believing that the numerical reduction of the vertebrae of Pipa to 8 is due to " excalation " of the second vertebra, as normally enumerated in other forms ; and the facts recorded by Portis2 for certain of the Tertiary Anura go far towards proving that the near ancestors of the So-called atlas of Siredon and Eana. Fig. 7. The so-called atlas of the Mexican Axolotl, Siredon pisciformis, from the side. Fig. 8. The same in Sana macrodon, exceptional. Fig. 9. The same in Eana catesbiana, exceptional. Figs. 10 & 11. The same in Eana esculenta, showing degrees in the occlusion of the foramen of exit of the trans-atlantal nerve. Fig. 12. The same in E. esculenta, normal. Figs. 7, 8, 10-12 enlarged. Eeferences as for figs. 3 to 6. living Anura possessed more than nine free vertebrae. While in one of the two transverse-process bearing Frogs' atlases herein described the nerve foramina (n, fig. 10) were, like those of Prof. Stewart's Rana catesbiana (n, fig. 9), widely open, in the other (fig. 11) they were, as in his R. macrodon (fig. 8), minute and reduced 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 1 Morpholog. Jahrb. Bd. vi. p. 297 (1880); cf, however, Adolphi, loc. cit. p. 315. 2 Atti Accad. del. Sci. Torino, vol. xx. p. 1173 (1885). |