?o8 



VERTEBRATE SKELETON 



in Mynnccophaga, elsewhere it is included in the vertical occiput. The parietals 

 form much or most of the cranial roof; the nasals are large, the premaxillae 

 reduced except in a few fossils, and Megathermm had one or two prenasal bones 

 between nasals and premaxilla;. Usually the lacrimal is small, the foramen in its 

 facial part. There is no foramen rotundum. The palatal processes of the 

 pterygoids are very large in Myrmecophaga, meeting in the middle hne and 

 carrying the choana; back to the level of the auditory meatus, a position paral; 

 leled only in Crocodilia and Cetacea. Tatusici has smaller palatal processes- 



FiG. 219. — Cranium of Myrmecophaga (Pouchet in Weber, '04). o, alisphenoid; 

 c, condyle;/, frontal; /, lacrimal; m, external auditory meatus; oc, occipital; p, parietal; 

 pi, palatine; ps, presphenoid; sq, squamo-sal; /, tympanic. 



they are lacking in other genera. In sloths the posterior end of the pterygoid 

 forms a bulla, connected with the tympanic cavity and with a squamosal cavity, 

 the walls of the cavity being largely membranous. Dasypus is noteworthy 

 in having a septomaxillary. The form of the lower jaw varies with the presence 

 or absence of anterior teeth, being respectively U-shaped or V-shaped, with a 

 long symphysis when viewed from above. The ramus is high in Glyptodon, 

 moderate in most other genera. 



Fig. 220.— Skull of Cholcepiis. /, frontal; /, lacrimal; ?«, maxilla; n, nasal: p, parietal; 

 sq, squamosal; 2, zygomatic. 



RoDENTiA (GUres) are characterized more by dentition than by cranial 

 structures. The low skulls have small cranial capacity, a roof largely of frontals, 

 the parietals small; a nearly vertical occiput and foramen magnum, and orbits 

 widely open to the temporal fossa;, there being no postorbital process from the 

 zygomatic arch, and one from the frontal only in squirrels and rabbits. The 

 Zygomatic arch is complete and usually is strong, although its composition varies. 

 In some it is largely maxillary and squamosal, the zygomatic bone being merely 



