142 STRUCTURE AND CLASSIFICATION OF BIRDS 



vomers are asgithognathous, but the maxillo-palatines are 

 united. 



As of desmognathism so of aegithognathism, PARKER 

 distinguishes four kinds 



Incomplete segithognathism (Hemipodes) is distinguished 

 by the fact that the broad double vomer has a septomaxil- 

 lary at each angle, which is only ' strongly tied ' to alinasal 

 cartilages. 



Complete Var. 1. The vomers are distinct from the often 

 long alinasal walls and turbinals : a small septomaxillary 

 appears on the angle of the alinasal cartilage, but does not 

 run into it. 



Complete Var. 2. Here the vomers are grafted upon the 



nasal wall. 



Compound, where in an aegithognathous palate desmo- 

 gnathism is produced by ankylosis of the inner edge of the 

 maxillaries with a highly ossified alinasal wall and nasal 

 septum. 



Among the higher families the septomaxillaries are often 



absent. 



It is clear, therefore, that a very narrow boundary line 

 separates desmognathism in some of its forms from schizo- 

 gnathism, and that there is a direct relationship between 

 desmognathism and aegithognathism. The only type of 

 skull which is really distinct is the dromaeognathous. 



In classificatory importance perhaps next comes the 

 condition of the nasal bone. (TARROD distinguished birds 

 into those with holorhinal and those with schizorhinal 

 nostrils. These terms refer to the posterior edge of the bony 

 nostril, which in one set of birds, the holorhinal, ends behind 

 with a clear oval outline (fig. 81), or in the schizorhinal 

 birds runs back as a gradually narrowing chink ; this latter 

 arrangement is shown in fig. 82. In the holorhinal bird a 

 straight line, drawn across the face from the posterior 

 boundary of one nostril to that of the other, passes in 

 front of the termination of the nasal processes of the 

 premaxilla. Tt is not always the case that a line drawn 

 similarly to that of the holorhinal birds passes behind the 



