THE SKULL. 597 



cipital bones of the higher types being merely identical in position 

 with the similarly 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 opisthotic, 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 

 indistinguishably 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 1 ) 

 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, leaving the 

 side walls of the skull almost entirely formed of membrane or 

 cartilage. 



In the ethmoid region there may arise a median ossification 

 forming the mesethmoid and lateral ossifications forming the 

 lateral ethmoids or prefrontals ; which may assist in forming the 

 front wall of the brain-case, or be situated quite externally to the 

 brain-case and be only related to the olfactory capsules. 



The labial cartilages. In most Fishes a series of skeletal structures, 

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

 mouth, and in connection with the olfactory capsules ; and these cartilages 

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

 form, in the higher types. They are more developed in the Cyclostomata 

 than in any other Vertebrate type. 



The meaning of these cartilages is very obscure ; but, from their being in 

 part employed to support the lips and horny teeth of the Cyclostomata and 

 the Tadpole, I should be inclined to regard them as remnants of a primi- 

 tive skeleton supporting the suctorial mouth, with which, on the grounds 

 already stated (p. 317), I believe the ancestors of the present Vertebrata 

 to have been provided. 



1 According to Kblliker there are two centres in Man in both the basisphenoid 

 and presphenoid. 



