DEVELOPMENT OF THE SKULL OF EMYS 741 



directed anteriorly. The lateral wall of this region exhibits an 

 irregular fenestra which indicates simply the beginning of the ab- 

 sorption of the cartilage under the influence of the adjacent squa- 

 mosum. The base of this portion of the quadratum opens freely 

 into the postero-dorsal portion of the pars articularis dorsal to the 

 incisura columellae. The pars articularis is continuous with the 

 pars mastoidea by means of the median wall and the dorsal mar- 

 gin of the latter which is rolled over laterally and ventrally to a 

 slight extent to enclose partially the concavity of this region from 

 above. The anterior and ventral margins of the anterior half 

 of the quadratum are rolled over laterally to form a rim enclos- 

 ing the tympanic cavity in front and below. Ventrally this rim 

 becomes somewhat heavier and higher to form the processus 

 articularis. The anterior half of the pars quadrata is much higher 

 than the posterior, so that the processus articularis projects well 

 below the ventral margin of the posterior part. Between the 

 latter and the ventrally projecting processus articularis in front 

 is a broad triangular sinus (incisura columellae) for the stalk of 

 the columella. 



The processus articularis bears a saddle-shaped articular sur- 

 face for the articulation of the lower jaw. The axis of the surface 

 is slightly oblique, sloping ventrally and posteriorly from the 

 medial to the lateral side. 



The pars palatina arises from the anterior end of the median 

 aspect of the pars quadrata near its ventral angle and extends 

 anteriorl}^ and ventrally as a gradually tapering rod, its distal 

 end bending latero-ventrally at a right angle. Of especial inter- 

 est in relation to the pars palatina is the processus ascendens. 

 This is a slender, laterally compressed process which arises from 

 the processus pterj'^goideus, about midway between its origin from 

 the pars quadrata and its sharp lateral bend. The processus 

 ascendens extends lateral to the foramen prooticum, but some- 

 what removed from it. Its free distal end is close to the ventral 

 end of the processus descendens of the parietale. Judged by 

 its relations to the nervals trigeminus and to the palatoquadra- 

 tum, it is the homologue of the 'columella' of the kionocraniate 

 lizards, as has been pointed out by Filatoff ('06). It extends 



JOURNAL OF MOKPHOLOGY, VOL. 23, NO. 4 



