98 



DE. B. A. BENSLEY ON THE EVOLUTION 



Eelative size of upper 

 posterior premolar. 



Species. 



I. Posterior premolar clispro- 

 portionately larger than 

 median 



II. Posterior premolar siigbtlj' 

 larger than or equal to 

 the median 



III. Posterior premolar smaller 

 than median and be- 

 comina; vestigial 



IV. Posterior premolar absent . 



Phascogale wiimtissiiim 



SmiiUhopsis hivtipes . . 



„ leucopiis . . 



\^Anteclnnomijs laniger.. 



Length of upper |No. of specimens 

 tooth-row. I measured. 



Sminthopsis crassicaudata 



„ murina .... 



„ macrotira 

 Pluiscogale flavipes 



,, minima .... 



,, Swainsoni . . , . 



„ calura 



i^ „ peiiicillata 



f PJuiscor/ale Wallacei . . . . 

 I ,, dorsalis . . . . 



<J ,, Thorheckiana 



I ChrHocercus criaiictiuda . 

 \^Dasyuroides Byrnei ... 



f Dasi/vrus hallKcatus 

 I „ Geoff rojfi 



<( ,, viverrinus 



I „ macidatus 



\^Sarcop]iilus ursinus 



2 

 1 

 2 

 1 



2 

 4 

 1 

 3 

 2 

 3 

 2 

 3 



1 

 2 

 1 



3 

 5 



7 

 2 

 2 



Like the carnivorous evolution of the molars, the reduction of the posterior premolars 

 is closely connected with increase in size of the animals, and the above table has 

 been arranged to show this by a comparison of the relative sizes of the upper posterior 

 premolars Avith reference to the median teeth, with the length of the upper tooth-rows. 

 The various species are here arranged in four groups in order to avoid unnecessary 

 description of the sizes of the premolars in each case. Within the different groups, 

 however, the forms have been placed where possible in their natural order. The lengths 

 of the tooth-rows have been measured in each case in a straight line from the bases of 

 the median incisors to the ends of the fourth molars. 



Two notable exceptions to this plan are fovmd in Phascogale apicalis and in 

 Smiutho2')sis macdonuellensis. In the former, two specimens measured showed the 

 length of the tooth-iows to be 18 mm. The posterior premolar is in an advanced stage 

 of reduction, so that the species presents a dental length characteristic of Group II. and 

 the premolar characters of Group III. A similar condition is found in the latter 

 species*, where, in animals of general proportions like the larger members of Group II., 

 the posterior premolar is greatly reduced. 



With regard to the premolar transformations in the later carnivorous stages, it has 

 already been mentioned that while during the reduction of the posterior premolar there 

 is a compensatory enlargement of the median tooth, there is subsequently a slight 

 reduction both of the latter tooth and of the anterior premolar. Neither of these teeth 



* Only spirit-specimens of this species and of CJia-tocercus and Dasi/woides were available, so that the tooth-rows 

 could not be cxactl}- measured. 



