DEVELOPMENT OF THE SKULL OF EMYS 707 



gradually from them as triangular plates. The comparatively 

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

 inences which for the most part follo^y the underhdng 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 ah'eady described. 

 The vestibular portion exhibits prominences corresponding to 

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

 the cochlear portion wliich remains simpler in its external form, 

 exhibits a flattened, oval, pocket-like form. 



The prominentia seniicircularis 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 seniicircularis 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 

 mediall}' from below and in front, so that its ventral end pro- 

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

 prominentia ampuUaris posterior forms a marked convexity on 

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

 terior semicu'cular cajial and dorsal to the crista parotica (cr.p., 

 fig. 28); the prominentia ampuUaris 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 utriculi, ventrally with that of the sacculus, pos- 

 teriorly with the prominentia ampuUaris posterior, and dorsally 

 with the prominentia semicircularis posterior. On the median 



