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period, which occurred when the ancestors of Hatteria possessed a 
dentition consisting of a definite number of teeth, equal in the young 
and adult, and undergoing regular displacement. This view first sug- 
gested itself to me entirely from a consideration of the tooth deve- 
lopment, but I afterwards found that it was supported also by several 
ot Denpr’s results (5) relating to the general development. DENDY 
describes the period of incubation as lasting about 13 months (an ex- 
tremely long period, requiring an explanation), and states that during 
the winter (say from the fifth to the ninth month of incubation — 
March to July) little progress is made, so that embryos taken from 
the egg in July are not much more advanced in development than 
those removed in March. His Stage R embraces embryos this 
period, and he states that those taken from the egg in March 
“almost exactly resemble in external characters those removed in 
July”. Later he says “by the time Stage R is reached the cur- 
vatures of the body have to a large extent disappeared, and the cra- 
nial flexure has straightened out, the head acquiring a marked Che- 
lonian aspect, with a conspicuous shell-breaker on the snout”. I have 
found this shell-breaker at a still earlier stage, in an embryo re- 
moved from the egg in January and therefore in the third month of 
incubation. In this embryo, the amount of ossification is very slight, 
and the dental lamina has not yet grown down into the mesoderm in 
the anterior region of the jaws. The only teeth indicated are’ those 
of the embryonic dentition, but few of which are as yet calcified. 
That such a transitory structure, which is of use only for the short 
time required to cut through the egg-shell, should be developed at a 
period 9 or 10 months before it is needed, is a remarkable fact. It 
is, indeed, inexplicable, except on the assumption that the more or 
less remote ancestors of Hatteria were hatched after 
some fourmonths incubation, the functional teeth being 
then the embryonic set of the present day. Another of 
Denpy’s discoveries which is of great importance is that of the em- 
bryonic colour markings. I quote briefly some of his remarks on this 
subject: “One of the most remarkable features of the development is 
the appearance at Stage R of a well-defined embryonic pattern on 
the integument, totally different from that of the adult. This pattern 
consists mainly of two distinct series of markings, 1) a series of nar- 
row, discontinuous, longitudinal stripes of white on a grey ground; 
and 2) a series of less well-defined, much broader transverse bands 
of white. At Stage S, shortly before hatching, the longitudinal striping 
has disappeared, except under the throat and chin, but the transverse 
