mi: skulks of mammalia. 



261 



the bones (Pt)< which obviously represent the pterygoids, articu- 

 late with them as well as with the basi-sphenoid. The anterior 

 and external edges of the pterygoids are united with an antero- 

 lateral prolongation of the pro-otic part of the periotic; and, 

 rather above the cleft between the latter and the pterygoid, is 

 fixed the large process of the malleus (m, Fig. 102), to which the 

 tympanic ring closely adheres. 



The periotic bone is remarkable for the lamellar prolongations 

 which it sends forwards from its pro-otic, epiotic, and opisthotic 

 regions, beyond the space required for the auditory organ, and 

 which enter more largely into the side walls of the skull than 

 any of its ordinary constituents. The periotic contributes 

 towards the floor of the skull by a triangular process, which it 

 sends in between the basi-occipital and the basi-sphenoid. Pos- 

 teriorly, it articulates largely with the ex-occipital, the foramen 

 for the eighth pair being situated between it 

 and the latter. By its wide superior prolonga- 

 tion it unites behind with the ex-occipital, pos- 

 teriorly and superiorly with the supra-occipital ; 

 anteriorly and superiorly, first with the parietal, 

 and then with a large bone (OS, Fig. 101) which 

 stretches outwards, upwards, and backwards from 

 the presphenoid and ethmoid, articulating partly 

 with the frontal, and more extensively with a&j 

 the parietal. Except in its unusual articula- 

 tion with the periotic, this bone corresponds Vo - 

 with the orbito-sphenoid. Between the superior pi- 

 prolongation of the periotic, and its thin and 

 imperfectly-ossified anterior and inferior pro- Bs \ 

 longation, there is an interspace filled up by 

 the squamosal. The lower edge of this pro- 

 longation articulates with the pterygoid, and, 

 in front of this, forms the upper boundary of 



BO 



BO 



the foramen for the third division of the fifth Fig-. 102 - — Under 

 nerve. Between its front edge and a small 

 process, sent up by the palatine towards the 

 orbito-sphenoid, is a small plate of bone, which alone seems to 

 represent the alisphenoid. 



half of the skull of 

 Echidna. 



