DEVELOPMENT OF THE SKULL OF EMYS 713 



circulare posterius extends laterally in a horizontal direction to 

 the septum semicirculare laterale, the septum semicirculare an- 

 terius to the lateral wall of the capsule. The floor of the cavum 

 vestibuU is very limited in extent because of the wide connection 

 ventrally between it and the cavum cochleae wliich lies below it. 

 The whole cavity of the capsule including that of the lateral semi- 

 circular canal is somewhat flattened from side to side. 



The cavum vestibuU may be differentiated for clearness in 

 description into a central portion with posterior, vertical, and ante- 

 rior Umbs. The posterior hmb is situated ventral to the septum 

 semicirculare posterius and encloses the ampulla posterior and 

 the median limb of the canaUs semicii'cularis lateralis as well 

 as the posterior portion of the utriculus (fig. 3, left side). The 

 vertical limb is situated between the anterior end of the septum 

 semicirculare posterius and the posterior end of the septum semi- 

 circulare anterius, and encloses the sinus superior utriculi (fig. 

 4). The anterior hmb is situated below the septum semicir- 

 culare anterius, and encloses the ampullae anterior and lateralis 

 and the recessus utricuU (fig. 6, left side). The central portion 

 of the cavity is situated medial to the septum semicirculare later- 

 ale and encloses the principal portion of the utriculus and the 

 recessus utricuh. 



The posterior limb of the cavum vestibuU is pear-shaped in a 

 vertical cross section, the narrow end being directed laterally and 

 dorsally to accommodate the posterior semicircular canal, while 

 the thickened end is medial and ventral for the accommodation of 

 the posterior ampulla. Posterior to the septum semicirculare 

 posterius, the orificium inferius canalis semicircularis posterioris 

 opens ventrally into the cavum vestibuU on its dorsal aspect. 

 Immediately ventral to this septum the canaUs semicircularis 

 lateraUs opens in a vertical longitudinal plane into the lateral 

 aspect of the posterior limb of the cavum vestibuU. DorsaUy the 

 cavum vestibuU communicates with the two vertical semicir- 

 cular canals by means of a common orifice situated between the 

 septa semicircularia anterius and posterius and near the median 

 side of the cavity. Opening into the anterior Umb of the cavum 

 vestibuU in a vertical plane facing obliquely forward and laterally 



