de'\'i;lopment of the skull jn the CEOCODILIA. 293 



This element contains the ampuUse 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 (vii, viii) ; 

 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 (fs.o). 



This swollen shell forms a sort of penthouse over the " meatus internus ;" the two 

 passages for the eighth nerve (fig. 7, vrii) lie obliquely under this part, and below and in 

 front of them we see the single hole for the facial nerve (vii). 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 epiotic 

 ends ; the rest of the outer surface is sinuous, answering to the membranous labyrintli 

 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 lahyrintJi in itself The facial nerve (vii) 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 (PI. LXIX. fig. 7, al.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 (v) ; in front the dilated base of the bone rises forwards nearly up to 

 the optic foramen (ii) ; then the cartilage runs over the optic nerve and joins the 

 postero-inferior angle of the orbito-sphenoid (o.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 fenestrse are relatively lesser than in the last stage (PI. 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 

 aheady become comjjound; for the basitemporals have united with its lower table 

 (PI. LXX. figs. 3—5, b.t, b.s). These are round shells of bone behind the pterygoid 

 wings; the bony scrolls for the internal carotids (i.c) lie over them. The true basi- 



