490 OSSIFICATIOyS OF THE CARTILAGINOUS CRANIUM. 



The front part of the cartilaginous cranium is ossified by a com- 

 plete ring of bone — the sphenethmoid bone — which embraces part of 

 the ethmoid region, and of the orbitosphenoid and presphenoid regions. 



In the Amphibia the cartilaginous cranium, with its centres of 

 ossification, is easily separable from the membranous investing bones. 



In the Amniota the cartilaginous cranium, whose development in 

 the embryo has already been described, becomes in the adult much 

 more largely ossified, and the bones which replace the primitive 

 cartilage unite with the membrane bones to form a continuous bony 

 cranium. 



The centres of ossification become again much more numerous. 

 In the occipital segment analogous centres to those of Teleostei are 

 again found ; and it is probable that the exoccipitals are homologous 

 throughout the series, the supraoccipital and basioccipital bones of 

 the higher types being merely identical in position with the simi- 

 larly named bones in Fishes. 



In the periotic there are usually three centres of ossification, first 

 recognised by Huxley. These are the prootic, the epiotic and opistho- 

 tic, the situations of which have already been defined. Of these the 

 prootic is the most constant. 



In Reptiles, the prootic and opisthotic frequently remain distinct 

 even in the adult. 



In Birds, the epiotic and opisthotic are early united with the 

 supra- and exoccipital ; and at a later period the prootic is also in- 

 distinguishably fused with the adjacent parts. 



In Mammals the three ossifications fuse into a continuous whole — 

 the periotic bone — which may be partially united with the adjacent 

 parts. 



In the pituitary region of the base of the cranium a pair of osseous 

 centres or in the higher types a single centre (Parker^) gives rise to 

 the basisphenoid bone, and in front of this another basal or pair of 

 basal ossifications forms the presphenoid, while laterally to these two 

 centres there are formed centres of ossification in the alisphenoid and 

 orbitosphenoid regions, which may be extremely reduced in various 

 Sauropsida, le^ing the side walls of the skull almost entirely formed 

 of membrane or cartilage. 



In the ethmoid region there may arise a median ossification form- 

 ing the mesethmoid and lateral ossifications forming the lateral eth- 

 moids or prefrontals ; which may assist in forming tlie front wa'l of 

 the brain-case, or be situated quite externally to the brain-case and 

 be only related to the olfactory capsules. 



The labial cartilaffes. I^^i most Fishes a series of skeletal structures, 

 known as the labial cartilages, are developed at the front and sides of the 

 month, and in connection with the olfactory capsides ; and these cartilages 

 still persist in connection with the olfactory capsules, though in a reduced 



* According to Kollikcr there are two ceiitre.s in Mar. in both the basisphenoid and 

 presplienoid. 



