FROM THE PURBECK OOLITE. 



651 



which to the living kangaroo-rat were immediately recognized by Dr. 

 Falconer on its first arrival in London.f 



No less than 10 species of the living genus Hypsiprymnus, com- 

 monly called the kangaroo-rat, and referred by Waterhouse to the Ma- 

 cropodidce, or kangaroo family, inhabit the prairies and scrub-jungle of 

 Australia, feeding on plants and gnawing scratched-up roots. A strik- 

 ing 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 7 vertical grooves (see /, fig. 1, where 

 the pre-molar of the recent Hypsiprymnus Gairnardi 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. 1 

 and 2, exhibit a no less striking similarity to Hypsiprymnus. Never- 



Fig. 2. 



»s' 



Plagiaulax minor, Falc. 

 (Magnified 4 diameters.) 

 All the teeth in this specimen are in place and well preserved. The hinder part of the jaw- 

 bone, with the ascending ramus and posterior angle, are broken away. 



a, i. Eight ramus of lower jaw, with all the teeth magnified 4 diameters. 



a. Incisor with point broken off. a', impression of same, showing that the inner side, near 



the apex was hollowed out in a longitudinal direction. 

 o. Offset of coronoid, the rest of which is wanting. 

 m. The two true molars. 

 p, rn. The four pre-molars. 

 c. The first molar, magnified S diameters. 



Upper figure, the crown. Lower figure, side view. 



d- %3cond molar, crown and side view. 

 e. Straight line indicating the length of the jaw, natural size. 



theless, 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. 1), indi- 

 cate a great deviation in the form of Plagiaulax from that of the living 

 kangaroo-rats. 



~ All the information concerning the natural history, osteology, and affinities 

 of Plagiaulax given in the following pages, is extracted from a more detailed 

 paper by Dr. Falconer, shortly to he published by the Geological Society, the 

 MS. copy of which has been liberally placed at the author's disposal. 



