220 THE DOG-FISH. 



and they are separated from each other by a thin 

 median internasal septum. 



b. The rostrum is formed by three cartilaginous bars, 



of which two run forwards from the upper surface 

 of the olfactory capsules, converging in front ; 

 while the third is an anterior prolongation of the 

 floor of the cranium, or brain-case. 



c. The anterior fontanelle is a large, almost circular 



hole in the roof of the skull, between and behind 

 the olfactory capsules. During life it is closed by 

 a connective-tissue membrane, in connection with 

 the inner surface of which is the pineal body. 



d. The supra-orbital crests are prominent curved ridges 



of cartilage running along the sides of the skull 

 from the olfactory to the auditory capsules, and 

 forming the dorsal boundaries of the orbits. 

 Along the inner side of each crest, between it 

 and the cranium, is a shallow longitudinal groove 

 in which lies a bundle of sensory canals. 



e. The foramina for the ophthalmic nerves are a pair 



of small apertures behind and to the outer sides 

 of the anterior fontanelle, and to the inner sides 

 of the supra-orbital crests. Through them the 

 ophthalmic branches of the fifth and seventh 

 nerves pass from the orbit to the dorsal surface of 

 the olfactory capsules. 



f. The auditory capsules are large lateral projections 



at the hinder end of the skull. They lodge the 

 auditory organs, certain parts of which can be 

 recognised without dissection. 



i. Between the two auditory capsules there is a 

 median depression, at the bottom of which are 

 a pair of holes. Each hole leads downwards 

 and outwards into a canal, the aquseduotus 

 vestibuli, which opens below into the internal 

 ear. 



