DEVELOPMENT OF THE SKULL OF EMYS 705 



terior direction from the section figured and which becomes con- 

 verted into a verj^ fine tube anteriorly by the growing together 

 of the walls of the groove dorsally. In the embryo modelled the 

 tube extends through only two or three sections and ends blindly 

 posterior to the cavum cochleae. It apparently is the can alls 

 hypoperilymphaticus which Nick ('12) has described in Dermo- 

 chelys, Chelydra, and Chelone; like that of the two latter it con- 

 tains no blood vessels but only a very loose connective tissue. 

 In an older embryo (carapace length of 13.5 mm.) this canal has 

 increased considerably in size so that it is one-third as large in 

 diameter as the canalis perilymphaticus and it communicates 

 anteriorly with the cavum cochleae at the most ventral and 

 posterior portion of the latter. While the canalis hypoperilym- 

 phaticus apparently is not differentiated at a much earlier stage 

 than that modelled, in an individual recently hatched of 28 mm. 

 in carapace length, the canal was large. 



The posterior wall of the cochlea, which is perforated by the 

 large, oval fenestra cochleae, bounds the fissura metotica in front. 

 This fenestra opens anteriorly into the cavum cochleae along the 

 median wall of the capsule and posteriorly into the extreme ante- 

 rior end of the sulcus supracristularis. The anterior end of the 

 sulcus supracristularis corresponds quite closely with the recessus 

 scalae tympani, as described by Gaupp in Lacerta. This region 

 communicates with the otic capsule by means of the fenestra 

 cochleae, with the cranial cavity by the extreme anterior end of 

 the fissura metotica, and opens widely throughout its extent to 

 the exterior of the cranium. These communications represent 

 respectively the fenestra cochleae (s. rotunda), apertura medialis 

 recessus scalae tympani, and apertura lateralis recessus scalae 

 tympani. 



The principal differences in the relationships of these three 

 openings in Lacerta and Emys lie in the fact that the plane of the 

 fenestra cochleae in the latter is vertical, while that of Lacerta 

 is horizontal, and also in that the front margin of the apertura 

 laterahs recessus scalae tympani Hes slightly behind that of the 

 apertura medialis in Emys. These slight differences are expUcable 

 by the fact that the cochlea in Emys has developed in a pos- 



