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upon the part played by the jaws when the teeth are worn down. 
The amount of jaw exposed is very considerable in later life, but even 
from the period of hatching the line of attachment of the epithelium 
to the bone is continually receding from the bases of the teeth. The 
result is that not only are the dentigerous edges exposed when the 
teeth are worn down, but long before this happens the lateral surfaces 
of the bones are exposed and are subject to the grinding of the op- 
posing teeth or jaw (Fig. 4). Were the superficial portion of the jaw 
to consist of ordinary bone, there is no doubt that the efficiency would 
be much impaired, and the pate of wearing away excessive. The 
whole of the exposed portions of the jaws are however, covered with 
a layer of more densely calcified bone, consisting of closely applied 
lamellae, and in its fully-formed condition containing no bone cor- 
puscles. It may be colourless or yellowish-brown, and in the process 
of grinding shews a marked tendency to develop straight cracks 
perpendicular to the plane of the lamellae. It produces the well-known 
polished appearance of the jaws of Hatteria, and has sometimes been 
described as enamel. As a matter of fact its resemblance to the latter 
substance is noticeable even in sections, especially where it occurs as 
a thin layer on the surface of ordinary bone, without any transitional 
region. This is the condition over a great part of the exposed sur- 
faces, but near the lines of attachment of the buccal epithelium to 
the lingual and labial faces of the jaws, it forms a thicker layer which 
can easily be traced into the ordinary bone. This specialized material 
begins its development some months after hatching, and is slowly 
formed as the epithelium recedes from the bases of the teeth. In 
most cases it is present as a thin layer round the bases of the latter, 
and may even pass up a short distance on to the enamel. 
There is no doubt that the production of the outer specialized 
layer on the surface of the jaws is an adaptation to special conditions. 
The reason for its production is to be found in the necessity for 
avoiding too rapid wearing away of the bone of the jaws, which are, 
as we have seen, greatly exposed. Here again, we can refer to the 
lengthening of the incubation period for an approximate explanation 
of the conditions observed. I have found that when teeth are shed 
in Hatteria, particularly those of the alternating series, the amount 
of bone that is lost at the same time is considerable (Fig. 6). Now 
when (in phylogeny) the teeth of the second dentition, instead of 
falling out to make way for the third, retained their positions on the 
bones of the jaws, the result naturally followed that the alternating 
series (second and third dentitions) occupied a different level from that 
