200 H. SPENCE!R HARRISON. 



of the tooth in tlie palatines^ for instance. This bony over- 

 growth assists in the "fusion" of the anterior teeth of both 

 upper and lower jaws, and its outer layers are similar in struc- 

 ture to the specialised bone described in the next paragraph. 

 A conspicuous feature of a prepared skull of Hatteria is 

 the polished appearance of the edges of the jaws for a con- 

 siderable distance above or below the insertion of the teeth. 

 Tomes found this outer layer to be true bone, but gives no 

 details of his preparations. In a ground section of the lower 

 jaw (transverse) I found the superficial layer to present the 

 appearance shown in fig. 29. The similarity to enamel is 

 striking, and this is in some places increased by its tendency 

 to fracture at right angles to the surface. I have found, 

 however, that weak acids do not dissolve it, and in some cases 

 I have been able to trace it into continuity with bone lamella 

 containing bone corpuscles. These are wanting in the fully- 

 formed material. In addition to this, it is not so highly 

 refractive as enamel, though more so than bone, and it seems 

 to be more affected by weak acids than does the latter. I 

 therefore conclude that it is a specialised layer of bone, 

 owing its similarity in appearance (in ground sections) to 

 enamel to a similar method of deposition by osteoblasts. It 

 apparently contains more calcareous matter than true bone, 

 and by its hardness enables the edges of the jaws to 

 functionally replace the teeth when these are worn away. 

 As this material is formed by the mesoderm (the process 

 commences at Stage T), the overlying epithelium moves 

 further from the teeth, until its line of attachment to the 

 jaws is separated from the bases of the teeth by a very con- 

 siderable interval. The intervening bony surface is covered 

 with this outer specialised layer. It is of interest to note 

 that in this retreat of the epithelium from the bases of the 

 teeth, the potential dental laminse are removed to a great 

 distance from the latter, so that the succession of teeth in 

 the adult could only be effected at the cost of an immense 

 absorption of bone, which is here very dense, particularly in 

 the mandible. Whether this has been a factor in the partial 



