176 



VERTEBRATE SKELETON 



unite to a common ethmoid. There is sometimes a separate centre 

 in the prenasal cartilage, sometimes the cartilage degenerates. At 

 lirst the ethmoid is visible on the outer surface of the cranium 

 between frontals and nasals, permanently so in ostriches; elsewhere 

 it becomes covered by growth from maxilla and premaxillae. 



Three bones develop in the otic capsule — pro-, epi-, and opistho- 

 tic (fig. 190) — and there may be other centres of uncertain homology. 

 The prootic extends far back on the lateral and medial sides, encloses 

 the seventh nerve and joins the opisthotic (mastoid) behind and the 

 alisphenoid in front above the foramen lacerum. The epiotic, ossi- 

 fying late, is small and fuses with the supra- and upper part of the 



Fig. 190. — Interior of cranium of ostrich (Pouchet et Beauregard, '89). a, alisphe- 

 noid; bo, basioccipital; e, ethmoid; /, frontal; 0+ eo, opisthotic and exoccipital; p, 

 parietal; pt, petrosal; r, rostrum; s, sphenoid; so, supraoccipital. 



exoccipital. The opisthotic arises between epiotic and exoccipital. 

 fusing with the latter and sometimes (gulls) entering the border of 

 the foramen lacerum. Vestibular and cochlear fenestrae lie between 

 pro- and opisthotic. All of the otica usually fuse to a petrosal 

 (periotic) bone which lies between basioccipital and basisphenoid 

 below, alisphenoid and squamosal in front, the latter largely over- 

 lapping it in front, and the parietal and occipitals above and behind. 

 The parietals, always small, arise laterally on the otic capsules 

 and eventually form the posterior part of the cranial roof, there being 

 in some birds a gap between them and the supraoccipital. Anteri- 

 orly they reach the frontals which form most of the roof and which 

 arise far laterally, and extend from the nasal roof back to the alisphen- 

 oids. Each has a broad ventral postorbital process extending down 

 so far that it forms a part of the orbito-temporal wall. Sometimes 

 this part ossifies separately and may represent the postfrontal of 



