153 
banding is still clearly recognisable on the back and tail”. Since this 
pattern is almost entirely absent in the adult, its development at such 
an early stage in the embryo is meaningless unless we assume that it 
was at one time the characteristic skin- pattern of the young animal 
at least, and if of the young animal at one period, then probably of 
the adult at a period still more remote. The lengthening of the in- 
cubation period may have been the direct cause of its present re- 
striction to the embryonic stages. 
We may now consider the probable steps in the modifications 
undergone by the dentition of Hatteria, proceeding on the assumption 
that at one period its ancestors were hatched after about four months 
incubation, having then a dentition comprising on each side, about 
= marginal teeth, and 3—4 palatines; I leave the vomerine teeth 
out of consideration, owing to their present irregularity. The young 
animal had then an integumentary pattern of the character above de- 
scribed, and its teeth were subject to replacement by vertical successors, 
without increase in number. From some cause the incubation period was 
lengthened to the 13 months it occupies at present. The first dentition 
now became useless and was therefore shed before hatching. Its members 
probably have also diminished in size, and they have lost the connection 
with the bone to which the close proximity of, their enamel organs sug- 
gests that they were once attached. The bone itself was much retarded 
in its development and is now only present in small amount at the 
3rd month of incubation. The second dentition of the young animal, 
after the change took place, made its appearance at a stage when the 
animal was in the egg and had no need of it. The third dentition 
also now developed while the animal was within the egg and in the 
natural course of development should have displaced the second. The 
latter, however, consisted of teeth of a comparatively large size, and 
it would have been disadvantageous for these large teeth to be shed 
into the mouth of the embryo. The teeth of the second dentition were 
therefore retained and those of the third came up alongside them, 
thus producing an alternating series (Figs. 1, 2 etc.). The compara- 
tive numbers of the first three dentitions on each side are approx- 
5 9 20 
imately I =, Il and HI 5, 
only found one of the first dentition in the palatine, but there are 
apparently 6 or 7 of the second and third together. One of the re- 
sults of the compression of two sets of teeth into a small length of 
jaw is seen in the fusion of the alternating series in the lower jaw 
(Fig. 5). Why this did not occur in the upper cannot be said; in 
as regards the marginal teeth. I have 
