DEVELOPMENT OF THE SKULL OF EMYS 707 



gradually from them as triangular plates. The comparatively 

 simple prismatic form is greatly modified by a number of prom- 

 inences which for the most part follow the underlying parts of 

 the membranous labyrinth though not as closely as in Lacerta 

 because of the greater thickness of the capsular walls. 



The capsule may be differentiated into a dorsally situated ves- 

 tibular portion and a smaller, ventrally situated cochlear portion 

 which encroaches upon the basal plate, as already described. 

 The vestibular portion exhibits prominences corresponding to 

 the semicircular canals, with their ampullae, and the utriculus; 

 the cochlear portion which remains simpler in its external form, 

 exhibits a flattened, oval, pocket-like form. 



The prominentia semicircularis anterior (fig. 28) forms the 

 dorsal and antero-dorsal margins of the otic capsule. Its ventral 

 end, situated in the cupula anterior, widens to accommodate the 

 ampulla anterior which lies in a somewhat oblique position so 

 that it does not produce a marked convexity in the capsular wall. 

 The plane of the anterior semicircular canal inclines medially 

 from ventral to dorsal and from anterior to posterior. The prom- 

 inentia semicircularis posterior is continuous in front with the 

 anterior prominence and curves along the dorsal and posterior 

 margins of the capsule to the cupula posterior. The dorso- 

 median edge of this prominence is continuous for its middle third 

 with the tectum posterius. The plane of the canal inclines 

 medially from below and in front, so that its ventral end pro- 

 jects laterally much as does that of the anterior canal. The 

 prominentia ampullaris posterior forms a marked convexity on 

 the lateral wall of the capsule below the ventral end of the pos- 

 terior semicircular canal and dorsal to the crista parotica {cr.'p., 

 fig. 28); the prominentia ampullaris posterior accordingly forms 

 the postero-ventral margin of the capsule and bounds the ante- 

 rior end of the fissura metotica. On the median wall it forms a 

 continuous area with the gently bulging prominentia utricularis 

 which forms an area continuous in front with the prominentia 

 sinus superioris utricuh, ventrally with that of the sacculus, pos- 

 teriorly with the prominentia ampullaris posterior, and dorsally 

 with the prominentia semicircularis posterior. On the median 



