MORPHOLOGY OF TKLEOSTEAN HEAD SKELETON. 515 



this projects below, but lies quite close against the basis 

 cranii. 



Outside each raesotic ridge lies the floor of the auditory 

 capsule, which, owing to the disappearance of its fenestras 

 (fig. 2, /(?.'), is no longer divisible into parachordal and external 

 portions. The outer border of the capsule (fig. 8) has 

 remained unaltered anteriorly, but posteriorly it has grown to 

 the extent of forming a roof over the auditory and cranial 

 cavities — a process in which the occipital arch has taken very 

 little part. At the same time the ridge which separated the 

 two fenestrae has become moi'e marked ; whilst its posterior 

 pillar (cp. figs. 2, 8, au. c.') has grown up and fused with a 

 similar downgrowth from the roof. From the point at which 

 these meet, another pillar, over which the posterior semi- 

 circular canal runs, has grown back towards and united with 

 the occipital arch (fig. 8, dotted outline near fe."). The 

 whole complex presents internally essentially the appearance 

 seen in the adult (PI. 29, fig. 20). The capsule as a whole is 

 so moulded to the shape of the auditory organ that the course 

 of the semicircular canals can be traced even externally. Two 

 fenestrae (fig. 8, /e.") have been left in the side walls. 



The sphenoidal region is no longer a mere thickening of the 

 anterior auditory wall. The post-orbital process (fig. 3, s&.p.) 

 above is at its maximum development, and the pro-otic process 

 (o. 'pr.') below is broad, plate-like, and fused with the outer 

 border of the parachordal. Thus it completes the anterior 

 boundary of the foramen for the seventh nerve. In front the 

 trabeculae become increasingly flatter and broader, and are 

 fused along their contiguous margins in such a way as to 

 form a ridge above and a groove below. 



The ethmoid region (e.) is still rectangular, but as a whole 

 it has elongated, thus separating the pre-etlimoid (e. ^r.) 

 and parethmoid cornua (e. -p. c.) from one another more dis- 

 tinctly. The latter ai'e now expanded and wing-like, and pass 

 at their upper extremities into the supra-orbital bands [sh. h.). 



In association with the withdrawal of the pi'osencephalon 

 from this region, a median process-like upgrowth (e. m.) of 



