IvG DR. B. A. BENSLEY ON THE EVOLUTION 



of elongation and digitigrade development. In P. Bougainvillei (PI. 7. fig- 16) the 

 proportion in a specimen measured was found to be 1 : 11'3. Tlie hallux is now 

 i-educed almost to a tubercle. The second and third digits are more completely bound 

 to"'etlier and also more reduced ; they are set backwards in such a way that the notch 

 separating them from the fourth digit is placed considerably behind tliat separating the 

 fourth digit from the fifth. There is no hallucal or first digital pad. Tlie second and 

 third digitals are well developed and show faint signs of striation. The sole is hairy as 

 far forwards as the base of the hallux. 



In P. nasiita and P. Gnmii the conditions are more primitive, although the animals 

 are larger and approximate more closely to those of the short-footed series. A dried 

 specimen of P. nasiita showed a proportiou of 1 : 6'5. A spirit-specimen of P. Gunni 

 showed a proportion of 1:8. In the latter species the hallux is not so reduced as iu 

 P. Botigainvillei, and the second and third digits are not displaced backwards. The 

 characters are otherwise much as in that species. 



Thylacomys leuciira (PL 7. fig. 17) presents on advance on P. Bougainvillei. The 

 proportion of breadth to length is about 1 : 15. The two digital pads (morphologically 

 second and third) are fused together basally, but the compound structure thus formed 

 shows two small, smooth, apical areas representing the formerly striated surfaces. The 

 sole is completely hairy. Judging from a dried specimen, the pes of T. lagotis 

 corresponds closely with that of T. leucura just described. 



Chcpropns (PI. 7. fig. 18) presents a more advanced stage of the digitigrade development 

 tlian Thylacomys. In a dried specimen the proportion was found to be 1 : 21-4. The 

 sole is extremely narrow and completely hairy. The hallux is absent and the second 

 and third digits wholly vestigial. There is a large digital pad which is probably the 

 result of fusion of tlie second and third pads, as in P. Bougainvillei. It is an interesting 

 fact that while in P. Bougainvillei the second and third digits are placed behind the fifth, 

 in Chceropm they are placed in front. Thus, iu the reduction of the second, third, and fifth 

 digits, in connection with the monodactylous elaboration of the fourth, the recession, 

 as shown by P. Bougainvillei, first influences the second and third, but ultimately, as 

 shown by Choirojyiis, these elements are passed by the fifth*. 



* It is interestiug to notice, in connection with this form, the peculiar conditions which must have attended the 

 origin of its foot-modifications. In both the front and hind feet Chceropus presents digitigrade cursorial develop- 

 ments analogous to those of the placental Uugulata. The hind feet have become functionall}' monodactyl like 

 those of the Equidas in the perissodactyl series ; but whereas in the latter the third digit is the predominant member, 

 in Clun-opus it is the fourth. Dollo has already shown that the reason of this is that while in the Equidee the mono- 

 dactyl condition has proceeded from a normal pentadactyl one, in Chan-opus it has proceeded from a specialized 

 arboreal tyjie like that seen in the Phalangeridie, in which the third digit was already reduced and the fourth digit 

 predominant. The front feet present an analogous case. They are functionally didactyl, as in the artiodactyl 

 Ungulata; but whereas in the latter division the predominant digits are the third and fourth, in Chceropus they are 

 the second and third. The reason is that before becoming cursorial the foot of Chceropus was already specialized 

 in a fossorial direction. In the species of Perameles the first and fifth digits, and to a lesser extent the fourth, are 

 reduced, and the second and third enlarged for digging purposes. It is interesting to note that in Clweropus the 

 two functional toes are not exactly of the same size, the second being slightly but perceptibly smaller. 



In developing into a cursorial animal, Chixropns has been, in a sense, hampered by the possession of arboreal 

 characters in its hind feet and of fossorial characters in its front ones. 



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