158 THE VERTEBRATE SKELETON. 



attached to the tympanic membrane. The columella and 

 stapes are together homologous with the mammalian auditory 

 ossicles and with the hyomandibular of Elasmobranchs. Some- 

 times the term columella is used to include the whole ossicular 

 chain, the columella together with the stapes. 



The olfactory or nasal capsules (fig. 25, B, 12) are fused 

 with the anterior end of the cranium and differ from the 

 auditory capsules in being to a great extent unossified. There 

 are however two pairs of membrane bones developed in con- 

 nection with them, the vomers and the nasals. They are drawn 

 out into three pairs of cartilaginous processes, on the dorsal 

 surface into the prenasal and alinasal processes which bound 

 the external nares, and on the ventral surface towards the 

 middle line into the forwardly-projecting rhinal processes. 



The nasals (figs. 24 and 26, 11) form a pair of triangular 

 bones lying dorso-laterally in front of the fronto-parietals. 

 Their bases are turned towards one another and their apices 

 are directed outwards and backwards. They correspond in 

 position with the prefrontals of the reptilian skull as well as 

 with the nasals. 



The vomers are a pair of irregular bones lying on the 

 ventral surface of the olfactory capsules. Each bears on 

 its inner and posterior angle a group of minute pointed teeth, 

 while its outer border is drawn out into three or four small 

 slightly diverging processes, the two posterior of which form 

 the inner boundary of the posterior nares (fig. 24, B, 15). 



(3) THE JAWS. 



The upper jaw consists of a rod of cartilage connected 

 with the cranium near its two ends, but widely separated from 

 it in the middle. It is almost completely overlain by membrane 

 bone. With its posterior end the lower jaw articulates. 



The membrane bones of the upper jaw include first the 

 premaxilla, a small bone meeting its fellow in the middle 

 line, and forming the extreme anterior end of the upper jaw. 

 It gives off on its dorsal surface a backwardly-projecting 



