467 



sphenoidal rostrum and are not borne by the divergent posterior end of 

 the vomer. 



The maxillo-palatines, which are usually elongated, unite with the 

 anterior processes of the palatines and pass backwards leaving a fissure 

 between themselves and the vomer. 



The Desmognathous type. The vomer may be abortive or so 

 small as to disappear, or if present is slender and tapers to a point 

 anteriorly. The maxillo-palatines unite across the middle line either 

 directly or by means of ossifications in the nasal septum. The posterior 

 ends of the palatines and the anterior extremities of the pterygoids arti- 

 culate with one another and with the basisphenoidal rostrum. 



The Aegithognathous type. In this group the vomer is broad, 

 abruptly truncated in front and its deeply cleft posterior extremity em- 

 braces the basisphenoidal rostrum. The palatines have their postero- 

 lateral angles produced. The maxillo-palatines, slender at their origins, 

 extend obliquely inwards and backwards over the palatines and finish 

 beneath the vomer in expanded extremities that are not united either 

 with one another or with the vomer. Ossification may take place in the 

 part of the nasal septum in front of the vomer and the space between 

 it and the premaxilla may become filled with spongy bone, but there is 

 no fusion between this and the vomer. 



30* 



