chap, ix.j SKULL OF AMNIOTA. 331 



membranous bone at the side of the skull, which is 

 known as the squamosal in the abranchiate Verte- 

 brata. 



While in the Amphibia the posterior (occipital) 

 and anterior (ethmoid) portions of the base and sides 

 of the cartilaginous cranium undergo ossification, it is 

 riot till we reach the Amniota that we find the 

 central and lateral cartilaginous parts of the sphenoidal 

 region becoming bony ; when they do so we recognise 

 a basisphenoid, with an alisphenoid on either 

 side, and a presphenoid with corresponding later- 

 ally placed orbitosphenoids. Now, too, we can 

 distinctly see an ossified basioccipital, two ex- 

 occipitals, and a median supraoccipital, all of 

 cartilaginous origin, and surrounding the foramen 

 magnum. In the Saurcpsida the exoccipitals unite 

 with the basioccipital to form a single median 

 occipital condyle ; in the Mammalia the exoccipitals, as 

 in the dog, alone form the condyles, or some share is 

 taken by the basioccipital, but in either case the skull 

 is articulated to the vertebral column by two con- 

 dyles ; it is for this reason that some writers speak of 

 the Sauropsida as Monocondyla, and of the Mammalia 

 as Amphicondyla. 



At the anterior end, the cartilaginous plates which 

 subdivide the nasal cavity may undergo more or less 

 ossification, and give rise to the " spongy bones " 

 of the nose ; they enter into connection with tho 

 ethmoid in the middle line, and may become united 

 with the nasals (naso - turbinals) or maxillae 

 (maxillo-turbinals) at the sides. (See Fig. 189, 

 page 442.) 



In the walls of the cartilaginous ear- capsule there 

 appear centres of ossification, which are ordinarily 

 three in number ; of these the most constant is the 

 prootic, which alone is found in the Amphibia, though 

 in some fishes there are also epiotic and opisthotic 



