2 5 6 RLE ME NT A RY ANA TOM Y. [LESS. 



the process of development of human teeth are interestingly 

 illustrated by permanent conditions in other animals. 



Thus we may have, as in Sharks, an open groove along 

 each jaw, in which groove dermal papillae appear and undergo 

 calcification directly without their becoming enclosed in sacs 

 at all. 



We may have, as in the Pike, an enclosure of each papilla 

 in a sac, but no development of bone round it, the teeth 

 being attached to the jaw by ligament. 



We may have teeth which become anchylosed to the summit 

 of the jaw, there being no bony wall (or alveolus) developed 

 on either the inner or the outer side of the teeth, as in certain 

 Lizards (e.g. Psammosaurus\ termed Acrodont. 1 



FIG. 223. SIDE VIEW OF THE SKULL OF A LIZARD (Varanus}, WITH 



ACROPONT TEETH. 



a, articular bone of mandible ; c, coronoid bone of mandible ; d, dentary ; _/", 

 frontal; _/", pre-frontal ; f", post-frontal; /, lachrymal; m, malar; ;_r, 

 maxilla ; , nasal , o, pro-otic; p, parietal ; pt, pterygoid ; pt , columella, or 

 dismemberment of pterygoid ; pt" , os transversum ; px, pre-maxilla ; g, quad- 

 rate bone ; sq, squamosal. 



We may find a development of a bony alveolar plate on 

 one side (the outer), to which the teeth may become attached 

 by actual bony growth (anchylosis), as in the Iguanian 

 Lizards. Such a form of attachment is termed pleurodontA 

 We may find two alveolar plates of bone, but no transverse 

 bony partitions, as in Ichthyosaurus. 



We may find both alveolar plates together with transverse 

 partitions forming distinct alveoli, which nevertheless are 

 incompletely divided one from another at the hinder part of 

 the mouth, as in many of the Dolphin tribe ; and finally, we 



1 From uVpo , sharp, and oSoiig, a tooth, 

 a From ir\evp6v, a side, and odcnc;. 



