XXIV] TYPES OF PALATE 629 



them and the development of these muscles is an expression of 

 the habits of the animal a feature which is eminently subject 

 to adaptive variations. 



Such organs as the heart, the kidneys and the genital organs 

 are perhaps less variable than those we have just mentioned, but 

 these afford little or no help in classifying birds since their 

 structure throughout Aves is extraordinarily uniform. Another 

 objection to using internal organs as a basis for classifying Aves 

 is that the dissection of different types and the comparative 

 description of the internal organs have been so incompletely ac- 

 complished that according to one of the best bird specialists, the 

 external characters furnish just as reliable data as the ill-known 

 internal organs, In classifying we must endeavour to group together 

 birds showing as many points of agreement as possible and frankly 

 admit that the question of the affinities of many groups is at 

 present an unsolved problem. The points at present most relied 

 on in classification are the structure of the palate and the amount 

 of care given to the young. With regard to both these points we 

 recognise in some birds archaic features which they share with 

 the Reptiles. These enable us to separate a more primitive stratum 

 from a more advanced stratum of Birds. Huxley long ago showed 

 that four types of palate are found amongst modern birds which 

 are as follows : 



(1) The dromaeognathous palate. In this type the vomers are 

 large and flat and embrace the rostrum of the basisphenoid bone 

 behind and prevent the pterygoids and palatines from touching 

 it. The pterygoids are united to the basisphenoid by processes 

 termed basipterygoid processes. A palate such as this presents 

 considerable resemblances to the palate of Spkenodon. 



(2) The schizognathous palate. In this type the vomers 

 coalesce in front to form a pointed bone ; they are small behind 

 and the pterygoids and palatines rest on the basisphenoidal rostrum 

 and can play up and down on it. The maxillopalatine bones do 

 not unite in the middle line. 



(3) The aegithognathous palate. This type greatly resembles 

 the preceding but differs from it in that the vomers unite in front 

 to form a bluntly truncated bone and diverge behind. 



(4) The desmognathous palate. In this type the vomers have 

 the same form as in the second type but they are smaller and may 

 occasionally be absent altogether. The maxillopalatine bones are 

 united in the middle line so as to form a bony palate. 



