XIII 



PHYLUM CHORDATA 



519 



animal. The pterygoids (pt.) are in the form of flat plates con- 

 tinuous 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 united with the periotic. 

 The mandible consists of very narrow, styliform rami, which are 

 not firmly united at the symphysis. The condyle (cond.) is narrow, 



oc.cond 



Fio. 1128. Echidna aculeata. Ventral view of skull and right rarnus of mandible, a ng. angle 

 of mandible ; aud. oss. auditory ossicles ; cond. condyle of mandible ; cor. coronoid process ; 

 max. maxilla ; oc. cond. occipital condyle ; pal. palatine ; p. max. premaxilla ; pt. pterygoid ; 

 sg. squamosal ; ty. tympanic ring. 



rather more elongated antero-posteriorly than transversely. There 

 are very slight rudiments of the angle and of the coronoid process 

 (cor.). 



In the Platypus (Fig. 1127) the zygoma is stouter than in 

 Echidna, and there is a post-orbital process which is formed by 

 the jugal. The maxillary root of the zygoma develops a process 



