ORAL CIRRI OP SILUROIDS AND ORIGIN OF THE HEAD IN VERTEBRATES. 7 



A stylohyal {Sty. hy.) bears the ceratohyal {Hy.) which is massive 

 posteriorly. The thickened anterior end bears a hypohyal piece. 



Model of Sihirus (Figs. 3, 4). 



Sections 15 /j. thick. Every third section drawn, with magnification 

 28 (Zeiss Oc. 2, Obj. aa). Height of drawing above table 19 cm. 

 Wax plates 1 mm thick. Model, reconstructed as far as middle 

 of horizontal semicircular canal (on one side), 11 cm long and 15 broad. 



The drawings for this model were taken from sections belonging to 

 Prof. Dohrn, which he kindly allowed me to use. The cartilage was 

 at its maximum development, no replacement by bone having occurred. 



The skull wall, in the auditory region, is not completed on the in- 

 ternal aspect, but on the contrary, a large fenestra is left, as is the 

 rule in Teleostei. 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 cartilaginous roof 

 extends towards the middle line very slightly beyond the level of the 

 membranous labyrinth. The pterotic ridge, which is on a level with 

 the external semicircular canal, is not specially strongly developed. 

 Below the pterotic ridge is the articulation for the hyomandibular, 

 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 capsule 

 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 Siluroids, 

 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 prolongation 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 antorbital 

 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 con- 

 tinuous 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 



