THE SKULLS OF FISHES AND AMFHIIilA. 



205 



membranous fontanelle. The skull proper lias no such fonta- 

 nelle. There is a well-marked pituitary fossa, and the noto- 

 chord, very thick in the spinal column, tapers to a thread as it 

 enters the base of the skull, and ends belvind this fossa. 



The bones which are developed in relation with this cartila- 

 ginous cranium in the base of the skull are, — a great parasphe- 

 noid which extends back under the coalesced anterior vertebra?, 

 and forwards to the level of the nasal cavities ; and a slender 

 median bone in front of this, which underlies the rostral pro- 

 longation (e), and appears to represent the vomer. 



No distinct ossifications protect the lateral walls of the skull, 

 but the bones marked F (Fig. 82) send down processes for a 

 short distance, and the parasphenoid gives off transverse pro- 

 longations upwards and outwards, from each side of the middle 

 of its length, as in most fishes. 



The roof of the skull presents a number of distinct flat 

 ossifications, no one of which involves the subjacent cartilages, 

 and which vary very much in contour and extent in different 

 specimens. The general arrangement is, however, fairly repre- 

 sented by the accompanying figure (Fig. 82). 



Fig. 82. 



Fig. 82. — The cartilaginous skull of a Sturgeon, with the cranial bones. The former is 

 shaded, and is supposed to be seen through the latter, which are left unshaded. — a, 

 ridge fbimed by the spinous processes of the anterior vertebras, which have coalesced 

 with one another and with the skull ; b, lateral wing-like processes ; c, rostrum ; An, 

 position of the auditory organs ; Net, position of the nasal sacs. 



Of these bones, the pairs C, G and D, D clearly represent, 

 both in position and character, the parietal and frontal bones 

 of the Pike, while F, F similarly correspond with the squamosals 

 of that fish. 



In position, again, E answers to the ethmoid, H, H to 



