382 MAMMALIA OF MIDDLE PURBECK. [Oh. XX. 



portions of the skeletons of six new species of mammalia, as inter- 

 preted by Dr. Falconer, who first examined them. Before the begin 

 ning of the year 1857 the number of species recognized by the eminent 

 zoologist last mentioned amounted to seven or eight, exclusive of twc 

 which had already been found by Mr. Brodie and named by Professor 

 Owen. Before these interesting 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 portion 

 of the Middle Purbeck, most of them insectivorous or predaceous, 

 varying in size from that of a mole to that of the common polecat, 

 Mustela putorius. While the majority had the character of insec- 

 tivorous 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-jungie of Australia, feeding on plants 

 and gnawing scratched-up roots. A striking peculiarity 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 (see I, fig. 

 373, where the pre-molar of the recent Hypsiprymnus Gaimardi is 

 represented). 



The largest pre-molar in the fossil genus exhibits 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 has proposed for the fossil 'the generic name 

 of Plagiaulax. The shape and relative size of the incisor a, figs. 373 

 and 374, exhibit a no less striking similarity to Hypsiprymnus. 

 Nevertheless, the more sudden upward curve of this incisor, especially 

 in the larger species, as well as the number and characters of the other 

 teeth, and the shortening, compression, and depth of the jaw, taken 

 together with the backward projection of the condyle (d, fig. 373), 

 indicate a great deviation in the form of Plagiaulax from that of the 

 living kangaroo-rats. 



Our knowledge is at present confined to two fossil specimens of 

 lower jaws,* evidently referable to two distinct species, extremely 

 unequal in size and otherwise distinguishable. The largest, P. 

 Bechlesii (fig. 373), was about as big as the English squirrel or the 

 flying phalanger of Australia (Petaurus ' Australis, Waterhouse). 

 The skeleton of this phalanger (named P. macrurus, No. 1849, Mu- 

 seum of College of Surgeons) measures fifteen inches in length, 

 exclusive of the tail, which is more than eleven inches long. The 



* Three additional specimens of P. Becldesii have since been found, some with 

 the two back molars entire. They confirm Dr. Falconer's conclusion previously 

 expressed in regard to the affinity of Plagiaulax and Microlestes. 



