92 PEOr. W. K. PAEKEK OX THE STBUCTUEE OF 



form its moiety of the roof, which is concave in its fore part ; the frontal suture is 

 irregular ; and the hinder edge of the two bones forms a margin to the great fontanelle 

 ( fo). Each bone also is hooked postero-laterally, where the postorbital {ft.o) clamps 

 it ; and the hook turns inwards some distance over the side of the fontanelle : this part 

 is thick ; the inner part is thin and scale-like. The remaining three fourths of the large 

 bulging fontanelle is enclosed by a very narrow bar of bone on each side, each bone 

 having both outer and inner outgrowths; these bars are the parietals (^j). Each bar 

 is curved, fitting to the side of the swelling membrano-cranium ; in front they are 

 wedged in between the hinder part of the frontals and postorbitals {p-to) ; behind, 

 they meet each other at a moderate distance beyond the cranial cavity. Outside, each 

 bone at its middle sends outwards and forwards a small spur; the top of the squamosal 

 (sg) fits on to the angle thus formed. 



The somewhat dense parietal bones are developing a thin ragged tract of bone from 

 their inner edge ; behind, they are united by a growth of this kind — a tongue-shaped 

 tract, which runs forwards along two fifths of the sagittal line. Over the hind brain 

 (Plate XV. figs. 1, 3) this tract is already crested ; it is the beginning of the huge inter- 

 parietal of the adult (Plates XVI., XVII., & XVIII. , i.p). Thus this band of new bone 

 conjugates the primary paired parietals [jj) into one tract. Afterwards, when the post- 

 orbitals and squamosals meet over the temporal region, the parietals up to the ex- 

 ternal spike against which the squamosal i-ests (Plates XV. &. XVI. figs. 1 and 3, 

 p, sq) are absorbed, and the hinder part, becoming distinct from the huge interparietal 

 above, is ankylosed to the top of the squamosal below ; a trace, however, of the suture 

 can be seen in the adult (Plate XVI. fig. 1, p, s<j). 



Thus the single Jilial interparietal not only stops the growth of the two parental 

 parietals, but, like a lusty " sucker," draws half their life out of them, and carries them 

 up to a marvellous distance from their original position on the sides of the hind 

 cranium. 



Notwithstanding the rapidly-growing frontals, parietals, and interparietals, the roof 

 is uncovered over four fifths of its extent, as a large subcircular fontanelle split up in 

 its hinder two fifths. The marginal bones of the hind skull are so placed as to enclose 

 a lower temporal space {I. t. s), which is narrow as seen from above (fig. 3), and suboval, 

 widest above, as seen from the side. Still the width of the skull is considerably 

 increased by this outer basketwork of bones, which is finished above by the parietals 

 and interparietal. The postorbital (pt.o) is the broadest of these bones, and not only 

 forms a third of the orbital rim, but also sends backwards one broad process from its 

 upper part to bind the fore end of the parietal, and another larger and sharper snag, 

 which overlaps the jugal (zygomatic) process of the squamosal (sq). Its antero-superior 

 process runs over the orbit, but does not, now, meet the prefrontal (p-f) ; a large tract 

 of the frontal intervenes (figs. 1 and o,2Jt-0,f,p-f). 



