580 PROF. OWEN ON HYPS1PRYMNODON. 



from British mesozoic formations are polyprotodont * ; and most of the existing species 

 of that dental division of the Order are pentadactyle f . 



Among the Diprotodonts J the scansorial Marsupials retain the pentadactyle foot 

 with a maximized hallux §. In the gradatorial Diprotodonts, development has wrought 

 on the two outer toes (iv and v), and especially on the fourth, with ultimate loss of the 

 innermost or first toe. This modification, characteristic of the Potoroos, reaches its 

 extreme in Macropus and Choeropus. The instructiveness of the foot of Hypsiprymnodon 

 lies in its indications of an early stage of modification of the pentadactyle type in the 

 direction reached by the great Kangaroos. The fourth toe is, indeed, the largest 

 (woodcut, fig. 3, PL LXXII. figs. 6 & 7, iv), but not so much larger than the fifth (ib. v). 

 The size of both, being less exaggerated, shows the cleansing-digits (ib. ii & in) on a less 

 dwarfed scale. Finally, the hallux (ib. i) is not yet lost : it is small, indeed, and so 

 placed as hardly to be opposable to any other digit, unless to the metatarsal part of n. 



As the habit of progression on hard ground increased, the share taken by the fourth 

 toe, as being most on a line with or most directly continuing groundward the tibia 

 or main pillar of the leg, would determine a greater flow of blood to the moving 

 powers of that toe ; and as the exercise of these powers would become more strenuous 

 with the increase of weight to be so moved, especially when the motion was accelerated 

 by leaps, so the fourth toe has come, at last, as in the great Boomer Kangaroo, to be almost 

 exclusively the instrument applied to the resisting earth in the saltatory mode of 

 locomotion. 



The reduction of the toes in terrestrial birds follows an analogous course. I need 

 hardly premise that the fifth toe in the foot of the Lizard is not developed in any bird. 

 The first toe, or hallux, shows its opposable character and grasping function in the 

 Perchers. When retained in a ground-bird, Apteryx e. g. (PI. LXXII. fig. 11, i), it is, as 

 in Hypsiprymnodon, minute, and placed above the level of the other toes. In the Cassowary 

 (PL LXXII. fig. 12) it is lost ; the second toe (n) is reduced in length and breadth, but 

 is provided with an unusually long and slender claw, which the bird may be seen occa- 

 sionally to apply in preening the plumage ; the third toe (in) is that which has acquired 

 the largest size and takes the greatest share in the act of support and progression ; the 

 outermost toe (iv) is smaller, but not so reduced as the innermost (n, PL LXXII. fig. 12). 

 Here we have an analogous reduction to that shown in the Potoroo. 



Finally, in the Ostrich, besides i, the next toe (n) is lost ; and the swift and powerful 

 course of the bird is allotted, as in the Kangaroo, to the two outer toes, and mainly to 

 one of these (m), the outermost toe (iv) being much reduced, but still characterized by 

 its five phalanges (PL LXXII. fig. 13). 



A thaumatogenist || may, indeed, contend that the species Struthio camelus was 

 created, at the beginning, by primary power, with but two toes to each foot, in size, 



* ' Researches on the Fossil Romains of the Extinct Mammals of Australia, with a Notice of the Extinct Mar- 

 supials of England,' 4to, vol. i. pp. 12-75, plates i.-iii. (1877,). 



t Tom. cit. p. 105. + jfo^ p , j_q7. 



§ Art. Maesupialia, ' Cyclopaedia of Anatomy,' 8vo, 1841, p. 285, fig. 111. 

 || ' Anatomy of Vertebrates,' vol. iii. 1868, p. 814. 



