MOEPHOLOGY OF TELEOSTKAN HEAD SKELETON. 565 



behind the epiphysis :vud between the sphenoidal and 

 auditory regions. Again, it is not continuous cartilage 

 which makes up the floor and the side walls, but fontauelles 

 proportionately larger than those in the adult. In Lepi- 

 dosteus and Salmo at least there is no sign at any later stage 

 of a breaking down of walls to form fenestrse, but the mass 

 of the chondrocraniuni goes on increasing into old age, and 

 consequently, fontanelles maybe reduced or even obliterated. 



There is, in this connection, one feature about the develop- 

 ment of the stickleback to which particular attention may be 

 ])aid, viz. the fact that in it we have re{)eated ontogenetically 

 a breaking down of pre-existing cartilaginous palatine process, 

 suborbital bands, and trabeculte, such as might have taken 

 place phylogenetically. Consequently it is reasonable to 

 expect that, if a fenestrating process ever did take place, 

 such lowly forms as Lepidostens and Salmo should show 

 some developmental indications of it. 



At still earlier stages Gasterosteus, Atherina, Engraulis, 

 and Salmo also possess an anterior fontanelle homologous 

 with the " PrEefontalliicke," and separated from the posterior 

 one by the epiphysial cartilage. 



Thus the whole course of development points to an 

 ancestor whose cranium had two doi'sal fontanelles separated 

 by a band of cartilage usually related to the epiphysis ; also 

 lateral fontauelles for transmission of optic, trigeminal, facial, 

 and auditory nerves ; and a pituitary fontanelle. 



Further evidence from independent sources is to be sought 

 for in the Ostariophysi, in Polypterus, and the cartilaginous 

 Ganoids. 



The first constitute a branch of the bony fish series probably 

 separate from that to which the forms we have so far con- 

 sidered belong, and they exhibit all those features just 

 mentioned as probably characteristic of the ancestor. 

 Sagemehl thought that the epiphysial bar had remained in 

 order to serve as a support for the blood-vessels passing to 

 the brain ; but it could be said with equal plausibility that 

 this was the cause of its first appearance. 



