DEVELOPMENT OF THE SKULL IN THE BATRACHIA. 103 



The wide temporal regions become concave in passing into the orbital, and then tlie 

 skull narrows gently and bulges a little in front, near the ethmo-palantine " axils." 

 The ethmoidal region (eth.) is covered by a very short "anterior tegmen ;" the 

 margin of this part has a large evenly rounded edge, and so also, in the other 

 direction, has the "posterior tegmen," which is Umited to the superoccipital region, 

 and is barely twice the breadth of the narrow ledge in front. These two opposite 

 margins of the great single fontanelle (fo.) are very far apart ; the hinder is nearly as 

 far back as the middle of the auditory capsules ; the fore edge is nearly as far forwards 

 as the arcuate ethmo-palatine (fig. l,^xf.). 



The fontanelle, open over three-fourths of the cranial " barge," narrows in the 

 tempoi-al region, and only recovers three-fourths of its size behind ; the roof-bones 

 {J-})-) are at a considerable distance from it ui every part. 



The cartilage which bounds it laterally is narrow in front and widens in the 

 temporal region to thrice its fii'st breadth ; this is the lateral remnant of the tegmen 

 cranii, and it is, for the most part, quite independent of the walls (see fig. 2), which 

 from a little behind the ethmoidal " axils" to the back of the optic foramina (II.) are 

 entirely membranous. Thus the optic fenestra (the " foramen " is a hole through it) 

 is excessively large ; a good series of gradations in this respect are to be seen in the 

 Australian Anura, and will be described in due time. 



The floor of the skull is very narrow — only half the width of the roof — and is merely 

 composed of the trabeculse and an " intertrabecular " band of the same width. The 

 nasal regions are quite normal, except that the subnasal angle is simpler than in the 

 type. There is no "rostrum," and the pro-rhinals (p.rh.) turn inwards, and ai'e not 

 much dilated distally. The roof (fig. 1), widest behind, and the floor, widest in front, 

 are normal ; the septum (s.n.) is thick and clearly seen. 



The ossification of this little semi-membranous skull is of great interest, for its 

 very minuteness has enabled it to escape from the strict morphological bonds that 

 keep the larger kinds in order. 



Nearly aU the ossification of the endocranium is behind the orbits ; there are more, 

 and there are fewer, osseous centres than in the typical kinds. The additional centres 

 are the median bones that have been crushed out, so to speak, by the special law of 

 Batrachian morphology ; these have crept in agam. 



Fewer centres are seen in the lateral pails, for the occipito-otic centres (prootic and 

 ex-occipital, pr.o., e.o.) are here, as in Pseudis and other generahsed types, ossified fore 

 and aft, without the transverse dividing band of cartilage, seen in those in which the bone 

 begins only at the proper nerve outlets. And they are intensely ossified too ; only a 

 trifling edge of cartilage is left in the tegmen and in the floor (figs. 1 and 2), and in front 

 the bony matter takes up half the " alisphenoidal " tract, giving a good bony mai-gin to 

 the " foramen ovale " (V.). The upper and lower median synchondroses are of moderate 

 width ; the lower is oblong, and is less than a third the width of tlie inter-condylar 



