of the Fishery Board for Scotland. 



279 



front of the orbit, and forming a small upper part of the interorbital 

 septum. From its appearance in the shad, a part of the anterior end of 

 this would seem to correspond to a presphenoidal ossification. 



This interorbital median bone is closely surmounted by the cartilag- 

 inous roof of the skull. The upper and posterior part of the V-shaped 

 portion of this orbitosphenoidal ossification is bridged over by the pre- 

 viously described thin triangular sheet of cartilage (fig. 22, tr.cr.) which 

 lies over the thalamencephalon. 



A cross section of the skull through the orbits, looking back, shows 

 the anterior face of the cranial mass perforated by three relatively large 

 foramina (PI. XVIII. fig. 25); the upper for passage of the olfactory nerves, 

 bounded by the orbitosphenoids ; the median and largest, for the 

 optic nerves, having its lateral margins formed by the alisphenoids, 

 and its base by the transverse portion of the basisphenoid ; while 

 the lower foramen, within which is situated the pituitary body, is 

 roofed by the transverse part of the basisphenoid, floored by the 

 prootics, and has the vertical basisphenoidal spine passing downwards 

 across its face. 



The arrangement of, and excavations in, the bones composing the 

 cranium give to its inner aspect, when viewed from above, a peculiarly 

 irregular appearance. The anterior (orbitosphenoidal) floor supporting the 

 fore brain slopes backwards and slightly downwards to above the anterior 

 margins of the prootics. There it suddenly drops to the level of the 

 median portion of the latter, this anterior nearly vertical wall here being 

 formed by the alisphenoids laterally and by the basisphenoid mesially, the 

 main part, however, being open as the foramina already described. The 

 floor for the brain here is partially formed by the large spherical air 

 vesicles, in the posterior side of each of which is seen the somewhat ragged 

 narrow aperture through which the prolongation of the membranous 

 sacculi pass to their interior. From this point the floor extends back on 

 one level to the posterior vertical extremity of the skull, formed by the 

 ex- and supra-occipitals. This fairly wide anterior (prootic) floor for the 

 brain is contracted as it goes backwards till it forms a mere median point 

 posteriorly, by the pair of deep cavities containing the membranous sacculi 

 of the ear. These cavities extend downwards on each side, and are formed 

 principally by the ex- and basi-occipital and prootic bones. The bottom 

 of the cavities is largely occupied by the membrane-covered fenestra 

 already described. The floor to the brain is continued backwards from 

 the median posterior point of the prootics by the thin (internal) expanded 

 processes of the exoccipitals described in connection with these bones 

 as meeting in the middle line over the dorsal median ridge of the basi- 

 occipital. Forming anteriorly a mere point meeting the prootics, these 

 processes widen posteriorly, and extend upwards and outwards towards the 

 main body of the bone (PI. XVII. fig. 19; PI. XVIII. fig. 24. ex.oc), 

 overhanging the posterior part of the auditory sacculus, and forming a 

 curved rather triangular-shaped floor for the hind brain. Just within 

 their anterior margin is a foramen for the 10th nerve, which then passes 

 through the posterior corner of the ear cavity to again perforate the outer 

 (exoccipital) wall of the skull for its final exit. The large apertures 

 between the brain case proper and the latero-ventral cavities containing 

 the sacculi are covered by the large optic lobes. The median bony ridge 

 which interposes between the pair of ear cavities is formed principally by 

 that median portion of the basioccipital which forms also the roof of the 

 eye muscle canal. These muscles lie, therefore, below the level of the 

 sacculi, the bones around which do not show as enlargements externally. 

 The ear cavities are partially continued above the floor of the skull, and 



