274 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 11, 



stricted portion of the ramus between the bottom of the sigmoid 

 notch and the lower margin. In all the ferine animals, the pivot 

 of motion or transverse condyle is, for obvious mechanical reasons 

 connected with the functions of the jaw, brought on a short stem 

 close under the base of the coronoid process. In Plagiaidax it is 

 carried out upon a long pedicle behind, and, pro tanto^ there is a 

 great deviation from the predaceous type. The arrangement is 

 equally without a parallel among the herbivorous or omnivorous 

 types, in which the condyle is ordinarily elevated above the hori- 

 zontal plane of the teeth, with more or less freedom of lateral or 

 longitudinal motion. Further, the convex articular surface of the 

 condyle, and its vertical instead of transverse direction, are at variance 

 with the locked implantation of the jaw of a ferine animal. The 

 other leading indications all lean towards a vegetable feeder, namely, 

 the limited surface and moderate elevation of the coronoid above the 

 plane of the teeth ; the feeble development of the inflected margin, 

 and the absence of a thick angular process ; the advanced position 

 of the orifice of the dentary canal ; the offset of the inflected margin 

 above it, and the form of the symphysial suture. These characters, 

 taken in conjunction with the marked signification of the teeth, would 

 seem clearly to place Flag. Becklesii among the vegetable feeders. 

 In this view, the exceptional position of the condyle would be re- 

 garded as a special modification, having reference to the abnormal 

 character of the teeth, and the adjustment involved thereby ; i. e. 

 the excessive development of the premolars, and the suppression of 

 so large a portion of the true molars, together with the functional 

 degradation of the two which remain. 



Giving due weight to these various considerations, and with the 

 above-indicated analogy in the dental formula to guide us, I am led 

 to the conclusion that Plagiaulax may be regarded in the natural 

 system as a marsupial form of rodent, constituting a peculiar type 

 of the family to which Hypsiprymniis belongs, and as bearing, in 

 respect of number of teeth, the kind of relation to that genus which 

 Dromicia bears to the other Phalangers, and Acrohata to Petaurista. 

 Mr.Waterhouse includes the Kangaroo-rats among the MacropodidcBi 

 Plagiaulax could never be classed among the Kangaroos. But, al- 

 though inferred to have been allied to HypmjJi'yfnniis, the fossils were 

 generically widely distinct from the existing Kangaroo-rats. A great 

 many links of the chain which would place them in connexion are 

 unknown to us, some of which may yet turn up in the fossil state. 



The species of Plagiaulax must have presented a form of which 

 there is nothing to remind us among living marsupials. This 

 is indicated by the extreme shortening, compression, and depth of 

 the lower jaw, together with the sudden upward curve of the in- 

 cisor, and still more by the depressed position and backward pro- 

 jection of the condyle. For aught that we know to the contrary, 

 they may have had the volant habits of the Flying-Phalangers, 

 and flitted from tree to tree among the oolite forests by means of 

 parachute-folds of their skin. As the Kangaroo-rats are strictly 

 herbivorous, gnawing scratched-up roots, it may be inferred of 



