DEVELOPMENT OF TEETH IN HATTEKIA PUNCTATA. 199 



terminations, and being in many cases continued into the 

 enamel as far as its outer limit (fig, 28). This continuation 

 of the tubules into the enamel has been described in many 

 vertebrates, If a young tooth of Hatteria is treated with 3 

 per cent, HNOg in alcohol, the enamel is in the course of an 

 hour or two completely dissolved, and the dentine is then seen 

 to be surrounded by a fringe of short processes, representing 

 the ends of the dentinal tubules, perhaps still containing the 

 odontoblast processes. As will be seen in fig. 28, the enamel 

 is characterised by the presence of alternate light and dark 

 lines, running approximately parallel to the long axis of the 

 tooth. I have not isolated the enamel prisms, but from the 

 course of the dentinal tubules, from the marked tendency to 

 fracture at right angles to the surface, and from the ap- 

 pearances in early stages of development, I believe these are 

 at right angles to the surface of the tooth, if, indeed, they 

 exist at all as elements possible of isolation. 



The first teeth to fuse with the bone show the typically 

 acrodont condition, as seen in fig. 14 of a mandibular tooth 

 of Stage S. At later stages, however, secondaiy over- 

 growths of bone occur round the bases of many of the teeth, 

 which thus come to be firmly fixed in a socket with which 

 they are fused at all points externally, as well as at their base. 

 Howes and Swinnerton have used the term "hyperacrodont" 

 for this condition, and it is a very convenient one, inasmuch 

 as we are dealing with a type of attachment which is un- 

 doubtedly derived from the acrodont ; it would be inaccurate 

 to use here the term '^ thecodont,^' since although the base of 

 the tooth is in a socket, the origin of the socket and its rela- 

 tions to the tooth do not indicate a morjjhological identity 

 with the thecodont type. The bony upgrowth is of a rather 

 peculiar structure. It consists of a number of thin, closely- 

 applied lamella?, which in transverse sections of the decalci- 

 fied jaw give the appearance of a number of fibres converging 

 on the base of the teeth. The extent to which the upgrowth 

 actually surrounds the latter varies in different regions. It 

 may be conspicuously seen embracing a considerable portion 



