210 DE. B. A. BENSLEY ON THE EVOLUTION 



are as valuable as those of anatomical structure. From this poiiit of view the Marsupials 

 mav be arranged as follows : — 



1. Primary marsupial radiation. — Arboreal derivatives oi Metatherla. Jurassic ? Arctogieic. 



Jurassic Mammalia in part. (The identification of this group is wholly problematical.) 



2. ArctogcBic and Neogmic didelphyid radiation. — Derivative of ? 1. Oligocene — Lower Miocene. 



Didelphyidae, Microbiotheriidse ? in part. 

 8. Notogmc ?-af/ia/iort. — Derivative of 2. ? Mid -Tertiary — recent. Dasyuridse, Peramelidse, 



Phulangeridae, &c. 

 4. First Neogaic radiation. — Derivative of 2. Lower Miocene — recent {Canolestes) . Prothylacinidaj 



(Sparassodonta), Epanortbidse, Decastidagj Abderitidse, &c. 

 ."). Second Neogceic radiation. — Derivative of 2. ? Pliocene — recent. Existing Didelphyidae. 



The Systematic Arrangement of the Australian Families. 



lu the case of the Australian radiation the systematic arrangement which most 

 ncai'ly expresses the lines of family derivation is as follows : — - 



A. Diadactyla (second and third digits of the pes separate). 



Incisor formula ., ; upper molars with no postero-internal cusp (hypocone). 



Hallux reduced or absent Dasyukid^. 



1). Svndactyla (second and third digits of the pes conjoined). 



a. Dentition polyprotodont. 



Incisor formula -^; posterior premolars well developed. Upper molars 

 with paracone and mctacone separate, with hypocone (except Thylacomys). 

 Lower molars with well-developed talonid. Pes terrestrial, subdigitigrade ; 

 hallux vestigial or absent Peramelid.h. 



Licisor formula ^7,; posterior premolars absent. Upper molars with con- 

 joined pai-acone and metacone ; no hypocone. Lower vestigial talonid. 

 Pes terrestrial, plantigrade ; hallux well developed, not opposable, with 

 secondary claw Notoryctid.e. 



b. Dentition diprotodont. 



Pes arboreal; hallux well developed, fully opposable. Antemolar dentition 

 primitive. Anterior upper premolars always, and vestigial intermediate 

 teeth usually present. 



Functional incisor formula y Median lower incisors piercing or 

 trenchant. Molars rooted, brachyodout, quadrituberculate (except in 



Tarsipes), with bunoid or selenoid cusps Phalangerid.*. 



Pes terrestrial ; hallux not definitely opposable, reduced or absent. Ante- 

 molar dentition specialized. Anterior upper premolars and vestigial inter- 

 mediate teeth absent. 



Pes plantigrade. Incisors scalpriform. Median premolars absent. 

 Upper lateral incisors present. Molars hypsilophodont, rooted. Pes 

 with walkmg-axis external ; axial elements of tarsus enlarged. 

 Hallux retained as an internal balancer. Mesocuneiform fused 

 ■J-- with entocuneiform Du'iiotodo.vtid.e. 



