MAMMALS. 



357 



sthotics) squamosal, mastoid, and tympanic. These are frequently 

 fused. The tympanic, surrounding the external auditory mea- 

 tus may develop into a saccular auditory bulla. In front the 

 temporal gives off a zygomatic process which extends forward 

 to join the jugal, or, as it is called, the malar bone. 



An interparietal may be distinct, as in rodents, or it may 

 fuse with the supraoccipital, or, more rarely, with the parietals. 

 The frontals are usually paired, and in the ungulates they may 



e.n 



FIG. 346. Skull of young Tatusia, from Wiedersheim ; cartilage dotted. 

 aty, tympanic annulus; bhy, basihyal; chy, ceratohyal; cr, cricoid; d, dentary; ehy, 

 epihyal; en, external nares; eo, exoccipital; f, frontal; hhy, hypohyal; /', jugal; 

 in, incus; Ic, lachrymal; mk, Meckel's cartilage; ml, malleus; mx, maxillary; ;/, 

 nasal; occ, occipital condyle; /, parietal; pa, palatine;/.*-, premaxilla; sq, squa- 

 mous part of temporal; st, stapes; stm, stapedial muscle; so, supraoccipital; th, 

 thyroid cartilage; tr, trachea; II, V, passages of nerves. 



develop bony horns. Frequently each gives off a postorbital 

 process which, approaching or meeting the jugal, partially or 

 completely separates the orbit from the temporal fossa. Post- 

 frontals are lacking. 



The cranium is closed in front by the ethmoid bone in which 

 may be recognized a median (mesethmoid) plate which divides 

 the nasal cavity into right and left halves, and on either side a 

 lamina cribrosa perforated for the passage of the olfactory nerve. 



