XIII 



PHYLUM CHORDATA 



519 



animal. The pterygoids {j)t) arc in the form of Hat phites con- 

 tinuons 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 periotio. 

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

 not hrndy united at the symphysis. The condyle {vond.) is narrow, 



oc.cond. 



cohfi 



citcd.css 



Kmcuic\ 



Firi. 1128. — Echidna aculeata. Ventral view of skull and right ramus of mandible, iinxj. angle 

 of mandible ; avil. '<.«. auditi'ry ossicles ; cond. condyle of mandible ; cor. coronoid process ; 

 max. maxilla ; oi\ cond. occipital condyle ; 'pal. palatine ; i>. max. premaxilla ; pt. pterygfiid ; 

 sij. squamosal ; ty. tympanic ring. 



rather more elongated antero-posteriorly than transversely. There 

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



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

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

 thejugal. The maxillary root of the zygoma develops a process 



