640 



MAMMALIA 



[CH. 



formed by the exoccipitals and the basal one by the basi-occipital. 

 From this condition it is easy to see how the conditions in Birds 

 and higher Reptiles on the one hand and in Mammals on the 

 other may have been derived. Then the brain instead of lying 

 behind the eyes extends forward between and above them ; there is 

 consequently no interorbital septum, and the side walls of the brain- 

 case are thoroughly and 

 firmly ossified, not merely 

 represented by a vertical 

 plate imperfectly ossified, 

 as in a Bird, or nearly 

 entirely membranous, as 

 in some Reptiles. These 

 walls are in fact consti- 

 tuted behind by bones 

 termed alisphenoids 

 which in reality corre- 

 spond to the epipterygoids 

 of Reptiles (see p. 572). 

 These "alisphenoids "are 

 applied to the wall of 

 the cranium above but 

 below they diverge out- 

 wards and join the ptery- 

 goid bone just as do the 

 epipterygoids in Lac&rta. 

 In front, the lateral walls 

 of the cranium are con- 

 stituted by orbitosphe- 

 noid bones, whilst a 



Fm.316. Dorsal view of the cranium of a Dog, strong mesethmoid 

 Canis famiiiaris x|. bone is developed in the 



1. Supra-occipital. 2. Parietal. 3. Frontal, internasal septum. This 

 4. Nasal. 5. Maxilla (facial portion). 



6. Premaxilla. 7. Squamosal. 8. Jugal, 

 10. Postorbital process of frontal. 11. Infra- 

 orbital foramen. 12. Anterior palatine 

 foramen. 13. Lachrymal foramen, il. First 

 c. Canine, pm 4. Fourth pre- 



13 



12 



septum is prolonged be- 

 yond the bones of the 

 face by a cartilaginous 

 plate forming the support 

 of a flexible nose or 

 muzzle ; this is a feature quite peculiar to Mammals. The base 

 of the cranium is completely ossified, not only behind by the 

 occipital and basisphenoid bones but in front by the presphenoid 



incisor, 

 molar. 



