XIII 



PHYLUM CHORD AT A 



527 



b.oc 



ex.oc 



a long, narrow rostrum, with the 

 external nares at its extremity. The 

 olfactory fossa3 are greatly developed. 

 The rostrum is composed of mes- 

 ethmoid, vomer, maxillae, and nasals 

 the premaxillas 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" (ptcr.\ in all but Cyclo- 

 turus, develop palatine plates. There 

 is no bony auditory meatus. The 

 mandible is entirely devoid of 

 ascending rarnus consisting of two 

 long and slender horizontal rami, 

 with a very short symphysis. 



In the Sloths (Fig. 1140) 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 premaxillaB are 

 small, and not firmly connected 

 with the maxilla?, so that they are 

 commonly lost in the macerated 

 skull. The jugal (Ju.) develops a strong zygomatic process which 



Ur 



na,s 



s.oc 



FIG. 1139. Skull of Anteater (Myrme- 

 cophaffa), ventral view. Letters as in 

 Fig. 1138. In addition, b.oc. basi- 

 occipital ; glen, glenoid surface for 

 mandible ; pter. " pterygoid." 



Fio. 1140. Skull of Three-toed Sloth (Bradypus tridactylus). Letters as in Fig. 1 



bifurcates behind into two branches, neither of which is connected 

 with the rudimentary zygomatic process of the squamosal, so that the 



K K 2 



