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PHYLUM CHORDATA 



497 



surrounded by bone behind, and so communicates freely with the 

 temporal fossa, which lies behind it. 



The periotic bones (pro-otic, opisthotic, and epi-otic) are not 

 separately represented in the skull of Mammals. Part of the periotic 



mass sometimes projects on the exterior at the hinder part of the 

 lateral region of the skull, and is the mastoid portion ; the rest is 

 commonly called the petrous portion of the periotic, and encloses 

 the parts of the internal ear the mastoid portion containing only 

 air-cells. The tympanic bone, which perhaps represents the 

 quadrato-jugal of Sauropsida, sometimes only forms a long tube, 



VOL. II 



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