The oral cirri of Siluroids and the ori^jin of tlie head in Vertebrates. 385 



The skull wiill, in the auditory region, is not comi)leted on the 

 internal asjject, but on the contrary a larjjçe fenestra is left, as is 

 the rule in 'I'eleostei. The cartilage of the; side wall of the cranial 

 cavity only extends back so far as to separate the anterior semi- 

 circular canal from the brain, while above, in the same region, a car- 

 tilaginous roof extends towards the middle line very shghtly beyond 

 the level of the membranous labyrinth. The pterotic ridge, which 

 is on a level with the external semicircular canal, is not specially 

 strongly ileveloped. Below the pterotic ridge is the articulation for 

 the hyoniaiidibular, which is a long narrow articulation, situated at 

 an angle of 30^ to the median axis. From the pterotic ridge, the 

 floor of the auditory i'ai)sule slopes, at an angle of 30" with the 

 horizontal plane, to the basicranial region where it is continuous 

 across the middle line, behind the large pituitary fontanelle. 



As is well known, an interorbital septum is never formed in Si- 

 luroids, and at this young stage the orbital walls are very wide apart. 

 The upper portion of the orbital wall (Parker's supraorbital band) 

 is a forward i)rolongation of the auditory capsule, and is triangular 

 in section for some distance, the pterotic ridge being also continued 

 forwards. The pterotic ridge can in fact be traced on to the ant- 

 orbital process. In front of, and partly below the auditory region, 

 is a great foramen in the skull wall , through which many of the 

 cranial nerves pass out, all from the optic to the facial. This is 

 partly filled, in the adult, by the so-called prootic. The floor of the 

 cranial cavity is continuous cartilage , except for the large pituitary 

 fontanelle, but, between this foramen and the pituitary fontanelle, the 

 cartilage is much reduced in breadth and thickness. The skull wall 

 in the anterior half of the orbital region is complete and passes into 

 the ethmoid region. At its anterior limit there is a foramen, the 

 Caoalis praeorbitalis (Pr. orb. c). At the junction of the orbital and 

 ethmoidal regions the epii)hysial bar {E/yh) passes across the supra- 

 cranial fontanelle, thus dividing the latter into pre- and post-epiphysial 

 portiims. 



The side wall of the skull is produced into a large vertical ant- 

 orbital process , separating the nasal and orbital cavities , and the 

 cranium is almost as wide in this region as in the auditory region. 

 At its anterolateral extremity, the antorbital process has an articu- 

 lation for the prepalatine i)iece iPrepal.). The nasal cavities are 

 wide apart, the rostral portion of the chondrocranium being very 

 broad. The olfactory lobes and nerves lie, as it were, in two short 



