THE SKULL. 



21 



Fig. 



S. 



Urostyle of Raiia 

 escuknta, seen 

 from the side, 

 twice nat. size. 



c Ventral border. 

 C.V. Canal, verte- 



braUs. 

 s Dorsal ridge 



(pr. «ijinos. ). 



V. The UiiosTYLE (Figs. 8 and 9). 



The urostyle is a long*, median, rod-like bone, which projects back- 

 wards, midway between the two hip-bones, and terminates over the 

 anus. The anterior end (Fig. H a) is the thicker and broader part 

 of the bone, and has two articular depressions (Fig. 9) for articula- 

 tion with the two facets of the ninth vei-tebra. The hinder end 

 is pointed and cylindrical, and terminates in a cartilage, which 

 is fixed in the tubular end of the 

 bone. The middle portion is almost 

 cylindrical, and has a groove along the 

 ventral surface which gradually becomes 

 less marked behind. The dorsal surface 

 bears a ridge (Figs. 8 and 9 s), which is 

 high and thick in front, becomes sharper 

 and less prominent as it proceeds 

 backwards, and gradually disappears 

 towards the hinder third of the bone, 

 so that in transverse section the an- 

 terior two-thirds of the bone appear triangular, 

 with a ventral and two lateral surfaces : while the 

 hinder third is cylindrical. The anterior portion 

 of the bone contains a canal, canalis vertebralis 

 (Fig*. 9 C.V.), Avhich is a continuation of the verte- 

 bral canal, along which the hindermost spinal nerves 

 pass. On each side of the anterior portion of the vxostyie oî Rana 

 urostyle are small apertures (Fig. 8 c.c), which lead from'thé ale, 



twice nat. size. 

 A bristle is 

 passed through 

 the canal, vert. 

 andoutthrongh 

 the canal, coc- 

 cyçi. of the right 



% 



into canals {canales coccj/fjei), which open into the 

 vertebral canal, and through which the coccygeal 

 nerves pass. In front of these openings and partly 

 overhanging them are small triangular projections 

 (Fig. 8) [jjrocessi/s transversarii) : these, however, are 

 not constant, and are more often found in H. escn- 

 lenta than in R, femporaria, in which latter species 

 the openings are smaller. 



I Anterior ex- 

 tremity. 



•.c Canal, coccyg. 



; Dorsal ridge 

 (pr. spinos. ). 



II. THE SKULL. 



The flat form of the frog's head, as in batrachians generally, 

 depends upon the wide separation of the jaw-bones of the two 

 sides, and on the large size of the orbital cavities and the horizontal 



