PURBECK FORMATIONS. 93 



still retaining that anomalous character of dentition which the small Alesozoic quadru- 

 peds manifested in a land geographically now that of England. 



So, with regard to the Plugiaulax with its defective molar formula, we must also 

 travel to the antipodean continent of Myriitecobius to find the forms most nearly allied, in 

 dentition, to the Purbeck genus. But even there they are no longer amongst the existing 

 Marsupials. 



The incisors of Thi/lacoleo (fig. 15, i) in size, position, direction, and shape closely 

 resemble those in Pla(/iaula<v, nuich more closely than does the lower incisor of any 

 poephagous, carpophagous, or rhizophagous species of Marsupial : the crown is completely 

 sheathed by enamel. But the laniary modifications are exaggerated or carried further out 

 mThylacoleo; the tooth is more compressed, its hinder trenchant edge is finely serrate, 

 as in Machairodus ; the crown is slightly recurved, as in Flacjianlax Becklesii and PL 

 Falconeri ; and the outer surface shows a very shallow and wide longitudinal depression 

 at its basal half. 



The last premolar (jy 4) is trenchant, and occupies two fifths of the longitudinal extent 

 of the dental series ; it is preceded by three small and 



early shed premolars ; it is followed, as in Pkifjiaulax, • ^^" 



by two small, tubercular molars (?« 1 and 2), which, 

 together, are limited to one sixth of the dental series. 

 The first of these, again, indicates the more strictly " ( 

 carnivorous character of Thylacoleo by the elevation 

 and compression of the fore part of the crown, de- 

 tractino; in the same degree from its triturating power Thyiacoieo carmfe.r, mandible and 



° O ° ^ teeth, ith nat. size. 



and character. The second lower molar is low, 



tubercular, one third the size of the first molar, implanted by one short thick root. 

 We know that the two small teeth succeeding the carnassial in the lower jaw were 

 opposed to a single transversely extended tubercular molar above, in Tliylacoleo. 



The large carnassial premolar in Thyiacoieo (fig. 1 5, /i 4) forms the same proportion of 

 the dental series as do the close-set three or four trenchant premolars in Plagiaulax. 

 The antero-posteriorly extended crown of the Thylacoleo's premolar has the enamel at the 

 basal part of the inner surface vertically and finely undulated, one cannot say grooved or 

 ridged. The worn margin demonstrates the trenchant or shear-blade mode of working 

 upon the similarly shaped and developed upper carnassial. The smooth even surface slopes 

 obliquely down the outer side of the lower and the inner side of the upper carnassial in 

 Thyiacoieo, showing the same relation of these teeth to one another transversely, as in the 

 Lion and other Pelines. 



In Tliylacoleo the outer wall of the mandibular ramus bulges out, as in Playiaiilax, 

 below the socket of the premolar. The outer crotaphyte depression is entire and imper- 

 forate. The dentary canal begins at a corresponding part of the inner or pterygoid 

 depression From the continuation backward and slightly downward of the lower border 



