P. BECKLESII AND P. MINOE. 425 



vegetable feeders. In this view, the exceptional position of 

 the condyle would be regarded as a special modification, 

 having reference to the abnormal character of the teeth, and 

 the adjustment involved thereby, i.e. the excessive develop- 

 ment 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 Hypsip- 

 rymnus 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 Acrobata to Petaurista. 

 Mr. Waterhouse includes the Kangaroo rats among the 

 Macropodidoe : Plagiaulax could never be classed among the 

 Kangaroos. But, although inferred to have been allied to 

 Hypsiprymnus, the fossils were generically widely distinct 

 from the existing Kangaroo rats. A great many links of 

 the chain which would place them in connection 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 incisor, and. still more by the depressed position 

 and backward projection 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 Plagiaulax that the 

 species were herbivorous or frugivorous. I can see nothing 

 in the character of their teeth to indicate that they were 

 either insectivorous or omnivorous. 



The larger species, PL Bechlesii, I have named after Mr. 

 Beckles, the discoverer, to whose energetic and well-con- 

 sidered explorations Palaeontology is indebted for so many 

 and important additions to the Upper Oolite (Purbeck) fauna, 

 after the efforts of the Geological Survey Department, specially 

 directed to the same object, under so able a head as the late 

 Professor E. Forbes, had proved unsuccessful. This species 

 equalled the size of a squirrel, or nearly that of Petaurus 

 macrourus, 1 one of the Flying Phalangers. The other species 



1 The skeleton so named in the Cat. (Osteol. Mus.) Koy. Coll. Surgs., No. 1849. 



