MAMMALIA OF THE PURBECK BEDS. 
327 
Fig. 304. 
Pre-molar of the recent Australian 
Hypsiprymnus Gaimardi, show¬ 
ing 7 grooves, at right angles to 
■ the length of the jaw, magnified 
8^^ diameters. 
Fig. 305. 
Third and largest pre-molar (lower 
jaw) of Plagiaulax Lccklesii, mag¬ 
nified 5i diameters, showing T 
diagonal grooves. 
Fig. 306. 
a no less striking similarity to Hypsiprymnus. IN^everthe- 
less, the more sudden upward curve of this incisor, as well 
as other characters of the jaw, indicate a great deviation in 
the form of Plagiaulax from that of the living kangaroo-rats. 
There are two fossil specimens of lower jaws of this genus . 
evidently referable to two distinct species extremely unequal 
in size and otherwise distinguishable. The Plagiaulax Pec- 
klesii (Fig. 306) 
was about as big 
as the English 
squirrel or the fly¬ 
ing phalanger of 
Australia {Petau- 
rus Australis, Wa¬ 
terhouse). The 
smaller fossil, hav¬ 
ing only half the 
linear dimensions 
of the other, was 
probably only one- 
twelfth of its bulk. 
It is of peculiar ge¬ 
ological interest, because, as shown by Dr. Falconer, its two 
back molars bear a decided resemblance to those of the Tri- 
sissic Microlestes (Fig. 389, p. 368), the most ancient of known 
mammalia, of which an account will be given in Chapter XXL 
Up to 1857 all the mammalian remains discovered in sec¬ 
ondary rocks had consisted solely of single branches of the 
lower jaw, but in that year Mr. Beckles obtained the upper 
portion of a skull, and on the same slab the lower jaw of an¬ 
other quadruped with eight molars, a large canine, and a 
broad and thick incisor. It has been named Triconodon from 
its bicuspid teeth', and is supposed to have been a small in¬ 
sectivorous marsupial, about the size of a hedgehog. Other 
jaws have since been found indicating a larger species of the 
same genus. 
Plagiaulex Becklesii, Falconer. Middle Purbeck. Right 
ramus of lower jaw, magnified two diameters. 
a. Incisor, b, c. Line of vertical fracture behind the pre¬ 
molars. pm. Three pre-molars, the third and last (much 
larger than the other two taken together) being divided 
by a crack, m. Sockets of two missing molars. 
