XIII 



PHYLUM CHORDATA 



495 



ex.oc 



of mesethmoid, vomer, maxillge and 

 nasals the premaxillae being very 

 small. The zygoma is incomplete, 

 and the orbit is not closed behind 

 by bone : the post-orbital processes 

 of the frontal being entirely absent. 

 The pterygoids (pter^) in all but 

 Cycloturus, develop palatine plates. 

 There is no bony auditory rneatus. 



i/ 



The mandible is entirely devoid 



t- 



of ascending ramus consisting of 

 two long and slender horizontal 

 rami, with a very short symphysis. 

 In the Sloths (Fig. 1071) the 

 cranial region is elevated and 

 rounded, the facial short ; the 

 frontal region is elevated, owing to 

 the development of extensive frontal 

 air-sinuses. The premaxillse are 

 small, and not firmly connected 

 with the maxillae, so that they are 



V 



commonly lost in the macerated 



/ 



skull. The jugal (fu.) develops a 

 strong zygomatic process which bi- 

 furcates behind into two branches, 

 neither of which is connected with 

 the rudimentary zygomatic process of the squamosal, so thatjthe 



icr 



Jb.masJc 



FIG. 1070. Skull of Anteater (J/V, /,/<- 

 cophaga), ventral view. Letters as in 

 Fig. 1069. In addition, b.oc. basi- 

 occipital ; gttii. glenoid surface, for 

 mandible ; ptavpterygoid. 



J 



FIG. 1071. Skull of Three-toed Sloth (i',v(/"/r<<,s tri'lactyhis). Letters as in Fig. 1069. 



zygomatic arch remains incomplete. There are, at most, the rudi- 

 ments of post-orbital processes of the frontals. The pterygoids 



