DEVELOPMENT OF THE SKULL OF EMYS 697 



From this embryo it is seen that the blastema of the otic capsule 

 extends medially slightly beyond the median wall of the mem- 

 branous labyrinth, so that the basal plate proper is greatly reduced 

 in width, and its lateral margins are shghtly concave antero-pos- 

 teriorly because of the position of the floor of the otic capsule. 

 The fusion between the floor of the otic capsule and basal plate 

 is complete in the stage modelled so that no line of demarcation 

 is visible. 



In general form the basal plate in the stage modelled may be 

 regarded as hexagonal, exhibiting short anterior and posterior 

 sides extending transversely, and antero-lateral and postero- 

 lateral sides. Extending along the mid-ventral line from in front 

 of the occipital condyle to the fenestra basicranialis posterior is a 

 low, rounded crest formed by the chorda dorsalis which in Emys 

 lies in the same plane with and enclosed by the basal plate and at 

 this stage is so large as to cause the latter to bulge both dorsally 

 and ventrally in the regions where the basal plate is quite thin. 

 Immediately behind the fenestra basicranialis posterior, the basal 

 plate on each side of the middle line exhibits a gentle convexity 

 on its ventral side which is caused by the extension of the pars 

 cochlearis as will be described later. The postero-lateral and 

 antero-lateral margins of the basal plate do not pass directly over 

 into the lateral portions of the occipital and otic regions, as in 

 Lacerta, but show a tendency to project freely laterally in the 

 form of crests. The postero-lateral margin projects laterally 

 and slightly ventrally as the crista inferior {c.i., fig. 1), whose 

 posterior end terminates freely at the side of the base of the con- 

 dylus occipitalis. This crest lies ventral to the anterior end of the 

 fissura metotica and foramina spino-occipitalia which thereby 

 come to open into a groove between the crista inferior and the 

 lateral portion of the occipital region. This groove, lateral to 

 which the ganglia vagi and spino-occipitalia lie, may be called the 

 'sulcus supracristularis' {s.s., fig. 1). Laterally the margins of 

 the basal plate project beyond the otic capsule, except in the 

 posterior portion of the latter, to form a crista substapedialis 

 {est., fig. 4), a horizontal shelf which is continuous anteriorly 

 with the crista basipterygoidea and on which the foot plate of 



