CERTAIN ABNORMALITIES IN THE COMMON FROG. 87 



The remaining vertebrae, although slightly asymmetrical, 

 present no remarkable peculiarity, except that the neural arch 

 of the ninth vertebra is feebly developed. 



Case B., PI. IV, figs. 9' — 12. — The axis and third vertebrae 

 both possess tubercles upon the transverse processes, probably 

 an indication of a tendency to bifurcation as seen in vertebrae 

 two and four of Case A. 



The ninth vertebra is abnormal, in that on the left side it 

 possesses a rudimentary, and on the right side a well developed 

 posterior zygapophysis, the latter overlapping the anterior 

 zygapophysis of a tenth vertebra. 



The general shape of the ninth vertebra and its large trans- 

 verse processes show that it is homologous with the ninth 

 vertebra of the normal vertebral column ; the vertebra behind 

 it, the tenth in the series, is therefore a portion of the urostyle 

 which has become segmented. 



This tenth vertebra has an imperfect centrum and only a 

 single anterior zygapophysis articulating with the zygapophysis 

 of the ninth vertebra. There are short pointed transverse 

 processes which are directed backwards. The neural arch is 

 complete but asymmetrical, and the neural canal is of consider- 

 able size (fig. 11). 



The centrum of the ninth vertebra has posteriorly two con- 

 cavities not two convexities as in the normal Frog, while that 

 of the tenth vertebra presents two convex articular surfaces 

 fitting into the two concavities in the urostyle. 



The Anura may present one or two pairs of transverse pro- 

 cesses placed one behind the other at the proximal end of the 

 urostyle (Disco gloss us). And this additional vertebra occur- 

 ring as an abnormality in the common Frog is interesting as 

 showing the potentially segmented character of the urostyle, 

 and its homology with the caudal vertebrae of Urodelous 

 Amphibia. 



The transverse processes on the atlas in Case A, and the 

 bifurcated nature of so many of the transverse processes seem 

 to indicate a tendency to increase the number of vertebrae, and 



