SECT. XIII 



PHYLUM CHORDATA 



487 



very narrow, and there is no rudiment of post-orbital processes. 

 The alveolar border of the maxilla (max.) is narrow and devoid of 

 teeth. The nasal and pre-maxillary region of the skull is drawn 

 out into a long narrow rostrum. Near the anterior end of this 

 is a rounded opening, the external nasal opening, which is entirely 

 bounded by the pre-maxillse the nasals not extending so far 



oc.cond 



cortd 



FIG. 1059. Echidna aculeata. Ventral view of skull and right ramus of mandible, anrj. angle 

 of mandible ; aiul. oss. auditory ossicles ; cotul. condyle of mandible ; cor. coronoid process ; 

 max. maxilla; oc. <'0ii</. occipital coiidyle ; pal. palatine; jj. max. pre-maxilla ; pi. pterygoid ; 

 sq. sqtiamosal ; ty. tympanic ring. 



forwards. An aperture in the nasal septum corresponds to an 

 actual perforation by which the nasal cavities are in direct com- 

 munication in the living animal. The pterygoids (pt.) are in the 

 form of flat plates continuous with the bony palate ; they extend 

 back so as to form a part of the walls of the tympanic cavities. 

 The tympanic (ty.) is an imperfect ring which does not become 



I I 2* 



