DEVELOPMENT OF THE SKULL IN THE CROCODILIA. 293 
This element contains the ampulle and part of the arch of both the anterior and 
horizontal canals (a.s.c, h.s.c) ; is perforated for the seventh and eight nerves (VIL, VIIZ) ; 
and is notched by the fifth nerve (v) and by the stapedial plate (st). 
Its fore margin is almost vertical, but its hind margin swells into the largest of the 
three shells of bone that bound the triradiate synchondrosis, Above, its oblique edge 
is finished by cartilage, where the alisphenoidal cartilage joins on to the capsule; below, 
it is separated from the basisphenoid by another similar tract. It binds on to a notch 
of the alisphenoid above, and then they are both notched to form the large foramen 
ovale (v). Postero-inferiorly there is an oblique notch a little higher up, finished by 
cartilage above ; this is the fore edge of the fenestra ovalis ( f3.0). 
This swollen shell forms a sort of penthouse over the “meatus internus;” the two 
passages for the eighth nerve (fig. 7, vimt) lie obliquely under this part, and below and in 
front of them we see the single hole for the facial nerve (vil). The bone under that hole 
is rounded ; it is scooped above and below the shell in front; this is all inside. Outside 
(Pl. LXX. fig. 11) there is a thin loop of bone above, where the air-cell of the eplotic 
ends; the rest of the outer surface is sinuous, answering to the membranous labyrinth 
within. Below, a crescentic wedge of bone grows obliquely forwards and outwards, and 
forms with the main plate a large open channel; this channel opens freely into the air- 
cavity inside the quadrate bone—a labyrinth in itself. ‘The facial nerve (vit) emerges in 
the top of this tympanic channel; the epiotic air-cell evidently ends in the crescentic 
sulcus between the thin loop of bone and the body of the prootic, supero-externally. 
The alisphenoid (Pl. LXIX. fig. 7, a/.s) is an obliquely oblong bone, leaning forward, 
somewhat, in front of the prootic, and locked by it above, the tooth of the latter 
fitting into a notch of the former. It is bordered by cartilage above and at all its four 
angles; its hinder margin is thick and pneumatic, the opening being into the great 
cavity in the basisphenoid. The fore margin is thin and scooped inside, and its oblique, 
almost straight free edge bounds the large upper “lateral fenestra,” which is now a long 
oval, with the broader end above. The postero-inferior edge is concave, to finish the 
foramen ovale (vy); in front the dilated base of the bone rises forwards nearly up to 
the optic foramen (11); then the cartilage runs over the optic nerve and joins the 
postero-inferior angle of the orbito-sphenoid (0.s). The hollow fore part of the ali- 
sphenoid is filled with the optic lobes; the bone is thin at that part, and bulges, 
correspondingly, outside. 
Both the lateral fenestre are relatively lesser than in the last stage (Pl. LXIX. 
figs. 6,7); but, with the exception of the basisphenoid, the rest of the endocranium is 
entirely cartilaginous, and differs but little from what is seen in the last two stages. 
The keystone of the inverted postsphenoidal arch is a very large bone, and has 
already become compound; for the basitemporals have united with its lower table 
(Pl. LXX. figs. 3-5, .t, 6.8). These are round shells of bone behind the pterygoid 
wings; the bony scrolls for the internal carotids (7.c) lie over them. The true basi- 
