116 DE. B. A. BENSLEY OX THE EVOLUTIOX 



Median and poste7'ior Premolars. — The modifications of these teeth are of interest as 

 showing much more clearly than those of the remaining teeth the sequence of species in 

 the present series and their general relations with the Potoroina3. It appears to he on 

 them that the dental change in this series has heen mainly developed. The evolution is 

 marked by the following characters: — {a) increase in size and functional importance, 

 with increase in the number of the grooves ; [b) gradual rotation of the cutting axis 

 inwards anteriorly. In Hypsiprymnodon (PI. 5. figs. 31, 34^) the posterior upper pre- 

 molars are comparatively short and their cutting-edges rounded. They are marked by 

 six prominent curved grooves ending in notches. In Bettongla jjenicillata (PL 5. 

 fig. 32 h) these teeth are more elongated, and the number of grooves is increased to 

 seven. The most interesting feature, however, is that their anterior portion has the 

 same characters as the wliole tooth in Hypsiprpnnodon, sliowing approximately the 

 same number of grooves and the same rounding of the edge. They have apparently 

 been derived from such a type as is seen in Hi/psiprpnnodon, simply by the lengthening 

 of the posterior portion. In B. Gaimardi, B. Lesueuri, and jEpyprymnus the anterior 

 portion of the cutting-edge is successively levelled, so that all parts of it come to project 

 to about the same extent. In B. Gaimardi the number of grooves is increased to eight, in 

 B. Lesueuri to ten, and in JEpyprymiMs to eight. Correlated with these modifications is a 

 rotation of the cutting-edge, by which the latter from being directed outwards anteriorly 

 comes to be directed inwards. The main features of this peculiar change are represented 

 in PI. 5. figs. 34, 35, 3G, where the upper teeth of the left side in Hypsiprymnodon, 

 B. peniciUata, and B. Gaimardi h.ave been drawn so that their cutting axes form tiie 

 same angle with reference to the border of the Plate that they do with reference to the 

 middle line of the palate. In Hypsiprymnodon the tooth is rotated outwards anteriorly, 

 so that the edge forms a considerable angle with reference to the middle line. In 

 B. peniclllala what has been referred to as the original or anterior portion of the tooth 

 is rotated outwards, as in Hypsiprymnodon, while the newer posterior portion is approxi- 

 mately parallel to the middle line. In B. Gaimardi the whole tooth is straight, while 

 in B. Lesueuri it is slightly rotated inwards, and in JEpyprymnus considerably so. 



It appears at first sight extraordinary that such a change as that just described, 

 involving a torsion of a sectorial tooth through an angle of nearly 90"^, should represent 

 a natural progression. The explanation becomes apparent, however, on comparing the 

 modifications of tlie muzzle in the different species. In ILy pslprymnodon the latter is 

 relatively long, but in the succeeding members it becomes gradually shortei' and broader. 

 At the same time the sectorial premolars become elongated antero-posteriorly. If under 

 both of these changes the premolars should retain their original outwardly rotated con- 

 dition, either their anterior and presumably most essential portion would have to be 

 supported on a weak projecting j^illar or their inner portion would have to encroach on 

 the palate. As it is, the rotation of the teeth in the opposite direction enables them to 

 be constantly supported directly in the line of the outer border of the jaw. 



The above descriptions of the upj)er j^osterior premolars apply almost equally to the 

 lower teeth. The median premolars, Mhich perform the same functions in the young as 

 the posterior teeth, replacing them in the adult, also show a progressive development 



