326 
ELEMENTS OF GEOLOGY. 
their non-existence on mere negative evidence.” Only two 
years after this remark w^as in print, Mr. W. R. Brodie found 
in the Middle Purbeck, about twenty feet below the “Cin¬ 
der-bed ” above alluded to, in Durdlestone Bay, portions of 
several small jaws with teeth, which Professor Owen recog¬ 
nized as belonging to a small mammifer of the insectivorous 
class, more closely allied in its dentition to the Amphitherium 
(or Thylacotheriiim) than to any existing type. 
Four years later (in 1856) the remains of several other spe¬ 
cies of warm-blooded quadrupeds were exhumed by Mr. S. 
H. Beckles, F.R.S., from the same thin bed of marl near the 
base of the Middle Purbeck. In this marly stratum many 
reptiles, several insects, and some fresh-water shells of the 
genera Pcdudina^ Planorbis^ and Cyclas^ were found. 
Mr. Beckles had determined thoroughly to explore the 
thin layer of calcareous mud from which in the suburbs of 
Swan age the bones of the Spalacotherium had already been 
obtained, and in three weeks he brought to light from an 
area forty feet long and ten wide, and from a layer the av¬ 
erage thickness of which was only five inches, portions of the 
skeletons of six new species of mammalia, as interpreted by 
Dr. Falconer, who first examined them. Before tllese inter¬ 
esting inquiries were brought to a close, the joint labors of 
Professor Owen and Dr. Falconer had made it clear that 
twelve or more species of mammalia characterized this por¬ 
tion of the Middle Purbeck, most of them insectivorous or 
predaceous, varying in size from that of a mole to that of 
the common Mustela putorius. While the majority 
had the character of insectivorous marsupials. Dr. Falconer 
selected one as differing widely from the rest, and pointed 
out that in certain characters it was allied to the living 
Kangaroo-rat, or Hypsiprymnus^ ten species of which now 
inhabit the prairies and scrub-jungle of Australia, feeding on 
plants, and gnawing scratched-up roots. A striking pecul¬ 
iarity of their dentition, one in which they differ from all 
other quadrupeds, consists in their having a single large pre¬ 
molar, the enamel of which is furrowed with vertical grooves, 
usually seven in number. 
The largest pre-molar (see Fig. 305) in the fossil genus ex¬ 
hibits in like manner seven parallel grooves, producing by 
their termination a similar serrated edge in the crown; but 
their direction is diagonal—a distinction, says Dr. Falconer, 
which is “ trivial, not typical.” As these oblique furrows 
form so marked a character of the majority of the teeth. Dr. 
Falconer gave to the fossil the generic name of Plagiaulax, 
The shape and relative size of the incisor, a. Fig. 306, exhibit 
