1864.. DAWKINS EHJSTIC BEDS AND WHITE LIAS. 411
and worn lower premolar of Hypsiprymnus Hunteri, magnified
four times, the latter being a little more advanced in wear than
the fossil. The outer and lower side of its crown presents but
the faintest traces of the plica?, while on the inside they are easily
recognized. It proves that the absence of plication in the corre-
sponding side (the lower) of the fossil cannot be admitted as evi-
dence against its Hypsiprymnoid character. In a word, the nearest
living representative of the fossil appears to me to be Hypsiprymnus
Hunteri, Bettongia rufescens, or some other of the Kangaroo-rats
with four-plicated premolars. All the premolars of Hypsiprymnus
which I have examined are at least twice the size of the fossil.
Figs. 3-5. — Ulustrating the affinities of Hypsiprymnopsis
Rhasticus.
Fig. 3.
Fig. 5.
Fig. 4 a.
Fig. 4 b.
Fig. 3. Premolar of Hypsiprymnopsis Bhceticus

X 4.
4 b. Right lower premolar of Hypsiprymnus minor, very much worn

X 4

= H. murinus of Hunt. Cat. 1784.
4 a. Unworn left lower premolar of H. minor

X 4.
5. Unworn left lower premolar of H. Hunteri (Ow.)

X 2.
Whether or not the tooth belongs to the Rhaetic Mierolestes cannot
be decided, because the tubercular true molars of the latter have
alone been found. Nor can its relation to Plagiaulax be affirmed,
as the worn crown reveals nothing of the verticality or the obliquity
of the folds. But, nevertheless, there is a sufficiently characteristic
portion of it left to indicate a premolar most closely allied to that
of the Kangaroo-rats. Until, therefore, additional remains be found,
I have provisionally named it Hypsiprymnopsis Bhceticus — a name
that represents its position in the geological scale and its zoological
affinities.
§ V. Range of Marsupials in Time.
It is a very significant fact, that all the Secondary Mammalia that
have with any accuracy been determined represent one or other of
the families of the most lowly organized Mammalian order*. Of

The small jaw of Sterecgnathus, from Stoncsfield, is sui generis,