STRUCTURE OF THE VERTEBRATES 



139 



the lower forms, due to the antero-posterior shortening. One 

 other structure is of phylogenetic interest. The pituitary opening 

 at the end of the notochord was described. From the dogfish to 

 man this pit becomes closed ventrally to form a pit in the floor 

 of the skull. In the mammal this pit is bounded anteriorly and 

 posteriorly by sharp ridges of bone and is the sella turcica 

 (Turk's saddle) in which the pituitary gland rests. 



Teeth of Vertebrates. The vertebrate teeth are homologous 

 with the dermal denticles of the elasmobranchs. In this group the 



Fig 76 Types of Tooth Attachment. (A) Acrodont, attached on biting 



margin of jaw; (B) Pleurodont. attached on mner side of jaw; and (C) 



Thecodont, the teeth sunk into cavities or thecae. 



Tooth 



Tooth 



Dentary 



Tooth 



A. Acrodont 



B. Pleurodont 



C. Thecodont 



Fig. 77. Cross Sections showing Tooth Attachment, 

 teeth are continuous with the denticles of the jaws, and are em- 

 bedded in the skin without any contact with the jaw cartilages. 

 The teeth are all alike and can be replaced indefinitely. 



With the development of bony jaws the teeth came in contact 

 with the bones, and fused to the tooth-bearing bones either (1) 

 on biting rims of the jaws, or (2) along the inner sides of the 

 bones. In the amphibia and primitive reptiles the teeth are all 

 similar, although some may be slightly longer than the others. 



The Crocodilia retain the tooth similarity, the homodont con- 

 dition, but show advance in having the teeth sink into the jaw 

 bones, fitting into sockets. The mammal-like reptiles also have 

 teeth in sockets, and other advances. In these reptiles the teeth 



