224 ON THE STRUCTURE OF THE SKULL. 



a separate centre, which subsequently coalesces with the supra- 

 occipital ; so that, just as the opisthotic in these animals ordi- 

 narily coalesces with the ex-occipital, the epiotic anchyloses with 

 the supra-occipital. 



In many Reptiles, though two of the three periotic bones 

 coalesce with their neighbours, the suture between the three 

 persists on the inner surface of the skull, and is always shaped 

 like a Y (Fig. 88, B) ; the stem of the Y answering to that part 

 of the suture which separates the pro-otic from the opisthotic 

 ossifications, while the diverging branches of the Y correspond 

 with the suture between the opiotic and pro-otic in front, and 

 that between the epiotic and opisthotic behind. In the Turtle 

 an interspace filled with cartilage takes the place of the stem of 

 the Y (Fig. 88, C). 



In the adult Crocodile the epiotic is united with the supra- 

 occipital, and the opisthotic with the ex-occipital ; but that 

 process of the opisthotic (c, Fig. 89, A) which separates the 

 fenestra ovalis from the fenestra rotunda (the anterior and inner 

 edge of which, only, is completed by bone) where it meets the 

 pro-otic below and anteriorly (at d, Fig. 89, A), sends down- 

 wards and backwards a process, which curves round the cochlea, 

 and, expanding to a broad plate, adjusts itself by harmonia (at b) 

 to the outer and lower edge of the opisthotic, and to part of the 

 posterior edge of the pro-otic. The anterior and inferior angle 

 of the broad plate is thicker than the rest, and is seen in the 

 interior of the dry skull, at the bottom of the stem of the Y- 

 shaped suture (*, Fig. 88, B). If, as has been remarked, this 

 part of the curved cochlear plate of the opisthotic be pressed 



cochlea rests in the fossa a, formed by the basi-sphenoid and basi-occipital. The upper 

 end, bounded externally only by cartilage, has disappeared in the dry skull and, with 

 it, the outer lip of' the fenestra rotunda, the plane of which is horizontal, and nearly on 

 the level of the dotted line leading from Op.O in the figure, d is a small process of 

 the pi'o-otic, against which the bend of the curved cochlear process (6) rests. The 

 dotted line from b indicates the position of the suture betweeu the hinder end of that 

 process and the remainder of the opisthotic bone. Ca, the carotid canal ; Eu, the 

 upper opening of the posterior of the two canals by which each tympanum communi- 

 cates with the common Eustachian tube. The narrow anterior tympanic canal opens 

 just in front of Ca, the cleft-like aperture being traversed by the dotted line from </. 

 In the Turtle's skull (B) Op.O is a distinct bone from E.O, and sends down a process 

 between /.o., the fenestra ovalis, and/.r., the fenestra rotunda, which terminates in no 

 recurrent hook, but otherwise corresponds exactly with the cochlear process (c) in the 

 Crocodile. 



