( 83 ) 
encircled by bone at its lateral border. In the partly-ossified skull 
of a young E. removed from the pouch, this fontanella even extended 
to the foramina rotunda, optica and spheno-orbitalia, which all eenfluated 
into one large vacuity in the lateral skull-wall. The same fact occurred 
in the skull of an Ornithorhynchus-foetus : the bony plate that was 
going to elose up this open space, was growing out from under the 
squamosal as a dermal bone. In this character it resembled the post- 
rontale, which is destined to anchylose with the orbito-sphenoïd. 
In consequence of this latter occurrence the orbital wings of the 
sphenoiad reach an enormous size in Monotremes. The presence of 
postfrontals in these animals, resembling those of Sauropsids, already 
mentioned by SEELEY, is proved beyond all doubt by the investigation 
of the skulls of new-born individuals. The orbital wings of the sphe- 
noid anchylose with the median corpus, in E. as well as in O., thus 
contrasting with the alisphenoïdplates in the former animal. 
HI. Petrosum. 
The petrous bone of O. is separated from the surrounding bones 
by three large perforations of the skull-wall; 1’ an anterior one, 
the foramen ovale, dividing it from the alisphenoïd, 24. a posterior 
one, the for. pro nervo vago et glossopharyngeo, separating it from 
the exoccipital (oec. later.); 84. a median one, through which no 
structures enter or leave the cerebral cavity and which separates 
the petrosum from the basi-occipital. 
In E. these holes are apparently wanting, but in reality they 
are all present, only they are lying much farther apart, and more- 
over the anterior and median ones are separated from the petrosum 
by the large pterygoid. The posterior opening is divided into two 
orifices, an anterior one which serves as an outlet to the nerves and 
is situated within the borders of the petrosum, and a posterior one, 
which is nothing but a fontanella, closing up in the full-grown 
animal and surrounded by the exoccipital. In O. the large size of 
the corresponding single opening is also due to incomplete ossifica- 
tion in the neighbourhood of the nerve-foramen. 
This scattered position of the three apertures around the petrosum 
in Echidna brings this bone into an all-round contact with other 
bones viz. the alisphenoid, pterygoid, basioccipital, exoccipital and 
squamosal. 
At the lateral wall of the skull the petrous bone of E. appears 
to be continued in a dorsal direction as a large patch of bone, but in 
