426 



PLAGIAULAX. 



was very much smaller, and, being one-half' the linear dimen- 

 sions, was probably about one-twelfth of the biilk of the 

 former, or near the size of the 'Pigmy Flying Opossum,' 

 Acrobata pygmoea. 



About the mammalian associates of Plagiaulax I abstain 

 from making any remarks beyond the few which are intro- 

 ductory to this paper, as the fossils will so soon pass, for a 

 detailed description, into the hands of Professor Owen, who 

 has already designated one of the largest of the new forms 

 by the generic name of Triconodon. The Purbeck mam- 

 malian genera announced up to the present date are, there- 

 fore, Spalacotherium, Triconodon, and Plagiaulax. 



There are, however, some points of general geological 

 interest, on which I may be permitted to make a few obser- 

 vations. 



The first is the relation of resemblance which the molar 

 teeth of Plagiaulax minor bear to those of the Triassic Micro- 

 lestes antiquus of Plieninger. The agreement in general form 

 is so close, that, had detached molars of both been met with 

 in beds of the same formation, they might have been taken 

 for back and front, or upper and lower teeth of the same, or 

 of nearly allied, species. The essential crown-characters are 

 the same in both, namely, two opposed longitudinal marginal 

 ridges, more or less lobed. or crenated, and separated by an 

 intermediate chasm or depressed disc. 1 A solution, however 

 approximative, of so ancient and obscure a mammal as Micro- 

 lestes is not devoid of interest. Plieninger considered it to 

 be predaceous, hence the name ; other naturalists were dis- 

 posed to regard it as leaning, however remotely, to the 

 omnivorous Pachyderms, or omnivorous Insectivora; while 

 Professor Owen, in recognizing at once the mammalian 

 character of the teeth, admitted them to be distinct from 

 anything fossil or recent known to him. 2 Pictet, in his 

 ' Paleontologie,' doubtingly includes Microlestes among mar- 

 supials, for reasons which he does not state, upon the autho- 

 rity of some of the German describers, whose memoirs I 

 have not been able to consult. Bronn notices Microlestes in 



1 Judging from the very careful draw- 

 ings and casts, the two teeth of Micro- 

 lestes, figured in Lyell's ' Manual of 

 Geology,' would appear, as there sur- 

 mised, to indicate at least distinct 

 species. The larger tooth (fig. 442, p. 

 343 of that work) resembles the penul- 

 timate molar of Plagiaulax Becklesii, 

 regarded in the side-aspect, inner sur- 

 face. There is in both an anterior 

 talon, forming an accessory lobule 



where it joins on with the anterior inner 

 tubercle. But I can detect nothing in 

 either like the basal cingulum referred 

 to by Mr. Waterhouse (loc. cit.). Fig. 

 441, representing the first discovered 

 tooth of Microlestes antiquus, crown 

 aspect, is the one which bears the closest 

 resemblance to the last time molar of 

 Plagiaulax minor. 



2 Cited in Lyell's Manual, 5th edi- 

 tion, p. 343. 



