260 



ON THE STRUCTURE OF THE SKULL. 



the skull that the lambdoidal suture is not very distant from its 

 summit. The plane of the roof of the skull slopes upwards and 

 forwards, from the occipital foramen to this point. 



Ficr. 101 



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no 



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AS 



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Fig. 101, 



-The skull of a young Echidna viewed from without, and in longitudinal and 



vertical section. 



The large parietals, anchylosed together in the middle line, 

 form all but a very small portion of the rest of the roof of the 

 skull, and are succeeded by the small frontals. These are met 

 in the middle line, inferiorly, by the lamina perpendicularis of 

 the ethmoid, which separates one olfactory chamber from the 

 other, and are united by sutures, anteriorly, with the long nasals. 

 These stop short of the anterior nasal aperture, being excluded 

 therefrom by the premaxillaries. 



In the base of the skull the basi-sphenoid, presphenoid, and 

 ethmoid are anchylosed together. The basi-sphenoid is a wide, 

 flattened bone, somewhat deflexed at the sides. Its long, thin, 

 postero-lateral margins articulate externally with the broad, flat 

 bones (Pt) which contribute above to form the floor of the cranial 

 cavity by filling up a vacuity which would otherwise exist between 

 the basi-sphenoid, periotic, and alisphenoid. The thick posterior 

 and external edges of these bones are excavated by a deep groove, 

 which forms the front wall of the tympanum and of the Eusta- 

 chian tubes. The palatine bones are completely anchylosed 

 with the sphenoid, and pass abruptly inwards from the outer 

 edges of that bone (Fig. 102). The anterior and internal edges of 



