174 PEOF. W. K. PAEKER ON THE MOEPHOLOGY 



alispkenoidal crest. Between this crest and the ear-mass the great trigeminal nerve 

 passes out through the foramen ovale (5). 



The synchondrosis seen in front of the bend does separate the orbito-sphenoidal {sp.e) 

 from the alisphenoidal regions ; hut the bony matter behiud the cartilage is all an 

 outgrowth from the prootic. This outgrowth is double, growing, as we saw, into both 

 wall and floor. The bony matter is not perfect in the inside (fig. 5) ; there is a 

 vertical band of cartilage directly in front of the foramen ovale (5), which broadens both 

 above and below. 



The optic foramen (2) is formed in the middle of a large membranous fontanelle, as in 

 the Frog; this space is margined with cartilage on the outside (fig. 3. 2). In front of 

 that ring there is solid bone up to the olfactory capsules (na) ; those right and left bony 

 walls correspond to the annular sphenethmoid of the Frog, but they are not conjugated 

 together, either above or below. The coping of this hardened wall is thick (fig. 4, sp.e), 

 but below it thins out and turns inwards. There is some floor to the skull iu front ; for 

 the internasal plate (i.n.c) is scooped, and the fore end of each bony wall is cemented to 

 the other by an elegant crescentic growth of cartilage, which has a thickened rim. 



The crescentic edge of the internasal plate below is of much smaller extent than the 

 upper, and extends further back; therefore there is some floor; on each side, just in 

 front of the bone, the olfactory crus escapes (figs. 4 & 5. 1). The large internasal plate 

 takes in most of the cornua below ; but the internal angle of each is developed into an 

 inturned tongue of cartilage, the " prorhinal " (c.tr) ; and between these two a similar 

 process grows from the mid line : this is the prenasal rostrum (p.n). These three out- 

 growths seem to bo the non-segmented rudiments of the paired and unpaired elements 

 of the foremost visceral arch, whose splints are the premaxillaries. 



The thickened internasal plate above is shaped like the letter X ; for its hind margin 

 is crescentic, its fore margin deeply notched in a rounded manner, whilst its solid middle 

 part is bounded by a crescentic ridge behind, and in front grows into diverging ridges, 

 the ridges to which the backs of the olfactory crescents are cemented. This structure 

 is eminently Selachian, but also foreshadows the nasal labyrinth of the higher Ver- 

 tebrata. 



The original crescent of cartilage closes round the outer nostril {e.n), and this is 

 further occluded by a membranous valve, which is fan-shaped, with the narrow part 

 looking; forwards. The nasal sacs are hollow below ; but the trabecular floor them to some 

 extent on the inner side. This part is invested by the huge vomers (v) ; but the ant- 

 orbital part, besides dipping down, has a new selvedge added to it. This is the cthmo- 

 palatine cartilage {e.pa), which is very apt to coalesce with the antorbital part of the 

 nasal capsule. 



The auditory capsule is nearly all solid bone ; but there is a cartilaginous tract outside 

 the ampullae of the anterior and horizontal canals (a.sc, li.se). This is a character of 

 considerable morphological import ; for this unossified tract was originally a growth from 

 the basal plate, and with this the otic process of the suspensorium coalesces. 



There is a well-ossified lip, like the mouth of a pitcher, to the fenestra ovalis : it looks 



