114 DE. B. A. BEKSLEY ON THE EVOLUTION 



stout, and curved. Tlie lower tooth is so sliort as to be scarcely caniniform, and bears a 

 small posterior accessory cusp. In P. JJoreyaiia we find both upper and lower teeth 

 greatlv reduced, the former showing usually small anterior and posterior cusps like 

 those of the premolars, and also a slight grooving of the root. The lower tooth shows 

 indications of a j^osterior cusp, and its root is also slightly grooved. A somewhat 

 similar condition is seen in Choeropus castanotis, excej)t that in this form there is an 

 anterior cusp on the lower tooth. Perameles Bougcdnvillel also shows a similar con- 

 dition, except tliat the upj^er tooth is not grooved. In P. nasiita both upper and lower 

 teeth are of a normal caniniform type ; they are, in fact, l^etter developed in this form 

 tliau in other members of the family, excepting the species of Thi/lacomys. In P. ohesula, 

 P. macrura, P. moresbi/ois/s, and P. Gunni the upper canines are short and curved, as 

 in P. Paffrayana. The lower teeth of P. moreshyensls and P. macrura are not so 

 caniniform as those of P. ohesula. They show a posterior cusp as in P. Maffrayaiia, but 

 in addition an anterior one. Apart from their stouter build, they thus resemble those 

 of P. Boreyana, P. Bougaiiwille'i, and Clia'ropus. In P. ohesula and P. Gunni the 

 lower teeth are rather short, but are otherwise normal. 



Apart from special phylogenetic considerations, the characters of the canines in this 

 series are of interest as illustrating the effects of retrogressive development. The 

 reduction in length, the addition of anterior and posterior basal cusps, and the grooving 

 of the root mark the return of a caniniform tooth to a premolariform one, from which 

 it originally evolved. The retrogressive characters of the Peramelidaj find a direct 

 parallel in Myrmecohius. It is interesting to note that retrogressive characters of a 

 somewhat similar kind are seen in Coenolesles {of. PI. 5. fig. 38) and in the Phalangeridae, 

 and further that in certain members (Dactylojjsila, Phalanger) of the latter family the 

 opposite development is illustrated, namely the conversion of an anterior premolar into 

 a caniniform tooth. 



Premolars. — Except for a slight tendency towards the reduction of the posterior teetli 

 in Choeropus castanotis, the premolars are well develoj)ed throughout the series. They 

 resemble in their general characters those of the Dasyurinse and Didelphyidye. Except 

 in Choeropus they increase in size backwards, and this character places the Peramelidae 

 in a more primitive condition than the Dasyurinte or even the existing Didelphyidae. 

 In tlieir relative positions the premolars show considerable variation. In cases where 

 the diastemata sepai-ating the teeth are of moderate leugtb, the latter show a j)i'onounced 

 tendency towards the enlargement of the basal cusps. The separation of the teeth being 

 due to the elongation of the muzzle, the characters show much the same relations in 

 the different species as those of the fifth upper incisors. 



Erom their association with the molars, the posterior premolars show indications of a 

 progressive evolution. In P. Boreyana and P. Eaffrayana they are laterally compressed 

 and of the usual primitive trenchant type common to the Dasyuridas and Didelphyidae ; 

 in the more specialized forms they become more massive and rounded in section, as in 

 the more advanced Phalangeridse. 



