the Skull of Diademodon. 323 



4. It has a posterior process passing backwards towards 

 the point of articulation of the lower jaw, which articulates 

 M'ith a process of the periotic and forms the anterior wall of 

 the tympanum. 



5. Lastly, the caroticus internus enters the cranial cavit\'^ 

 through a canal l)etween the posterior ramus of the ptery- 

 goid and the basisphenoid. 



The alisphenoid of Dasyurus has the following relations : — 



1. It articulates with the basispheno'd, 



2. It has a large ascending process which articulates with 

 the parietal, separates the exits of the IT., III., IV., V.^, 

 and VI. nerves from those of the second and third divisions 

 of the V. (which, however, pass through it and not behind it). 



3. It has an anterior part articulating with the palatine 

 and the " pterygoid,^^ which lies on its inner or ventral 

 surface. Internally it is separated from its fellow by the 

 vomer and the basisphenoid. 



4. It has a posterior process running back towards the 

 point of articulation of the lower jaw, which articulates with 

 the periotic and forms the anterior wall of the tympanum. 



5. Lastly, the caroticus internus enters the brain-cavity 

 through a canal lying between the posterior process and the 

 basisphenoid. 



Dasyurus is not exceptional in these relations, which are 

 found with only slight variations in all Marsupials, and from 

 which those of higher Mammals only diifer in the reduction 

 of the posterior process of the alis})henoid, which is, however, 

 found in a reduced form in many Insectivores. 



The relations of the pterygoid of Cynodonts (which are 

 essentially similar to those of the pterygoids of other primi- 

 tive Keptiles) are so exactly similar to those of the alisphenoid 

 of primitive Mammals, as described above, that the con- 

 clusion that the alisphenoid of JNIammals represents the 

 pterygoid of Cynodonts (and other Reptiles) is to me 

 irresistible. 



This conclusion is, I believe, new; but Parker and Cope 

 have held that the epipterygoid of Reptiles was the alisphenoid 

 of Mammals. 



So far as I know, the only evidence against the view that 

 I have just urged is that the pterygoid of Reptiles is a 

 membrane bone, whereas the mammalian alisphenoid is a 

 cartilage bone. This difficulty does not seem to me a very 

 serious one : the tendency nowadays is to lay less and less 

 stress on this difference, and in any case the whole ptervgoid 



