AKDITOKY HKCTOX I\ J)K'V.\()n()N. 421 



Th*» semiciirular cMuals iire lost fioiu tlie left siilo of the 

 Kpeoiiuen, and though they are preserved on the right side, as they 

 are {)artly filled with calcite and the bone is much epidotiseil, it is 

 iin[x)ssil>le satisfactoi-ily to decalcify the right side. Still, sufficient 

 litis l)een successfully accomplished to show at least a pait of each 

 ciiual. 



The posterior semicircular canjil lias a distinct ampulla, which 

 lies very close to the canal for tlie ixth, xth, and xith nerves. 

 From here it passes upwards and forwards, then inwards, and 

 lastly downwards, doubtless uniting with the upper end of the 

 anterior canal. 



The horizontal canal, tliougli well shown in its middle region, 

 has neither its anterior nor ])osterior ends displayed. The position 

 of the canal will be better understood from tjie figure than from 

 description. 



The anterior semicircular canal is badly dis[)layed. It runs 

 mainly upwards to unite with the descending part of the posterior 

 canal . 



The cast of the posterior ])orti{)n of the cranial ca\ity and of 

 the basicranial nerves is beautifully preserved. 



Behind the lower end of the posterior semicircular canal is seen 

 the cast of the large canal for the ixth, xth, and xith nerves ; 

 where it leaves the cranial cavity it is slightly extended antero- 

 posteriorly, but where it leaves the skull it is flattened a little 

 vertically. 



The canal foi- the xiith nerve passes down a short distance 

 behind the jugular foramen, and unites with it befoi-e it leaves 

 the skull. The fact that the xiith nerve has only a single canal 

 is interesting in view of the fact that in the Cotylosauria, as 

 exemplified by Pareiasaurus, we have two foramina for the xiith 

 nerve, and a similar condition is found in the Cynodontia and in 

 the Marsupialia. In the Pelycosauria, as has been shown by Case, 

 and in the Eutheria, there is, as in the Anomodontia, only a 

 single foramen for the xiith nerve. 



The canal for the viith nerve lies a little in front of the 

 vestibule. It is long and narrow and passes downwards and 

 slightly outwards, and ends in the small foramen seen at the 

 bottom of the deep groove between the back part of the basi- 

 sphenoid and the ptei-ygoid. 



In front of the specimen is seen the deep pituitary fossa, and 

 below it the carotid canal. The latter begins at a small rounded 

 foramen near the front of the postei'ior processes of the basi- 

 sphenoid. The main pai't of the canal passes upwards, forwards, 

 and slightly inwards, and unites with the canal of the opposite 

 side just in front of the hypophysis. Fi'om the lower end of the 

 canal is given off a smaller canal, which passes upwards and 

 somewhat more forwards. While the large canal is pretty 

 certainly that for the carotid artery, the smaller is possibly for 

 the arteria opJdhalmica. Whatever be the condition of the 

 arteries within the cranial cavity, it must have been very uidike 

 that of Sp]ii;nodoii or other living reptiles, and though perhajs a 



