432 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



to the preceding, Stegosaurus. This animal had for its immediate 

 ancestors bipeds like the ancestors of Triceratops, and, like Tricera- 

 tops, it became readapted to quadrupedal life. But while the tri- 

 radiate pelvis of its far-away quadrupedal ancestors has been physio- 

 logically reestablished through regressive evolution (atrophy) of 

 the postpubis and ischium in Triceratops (Dollo, 1. c. p. 446) these 

 parts have evolved in a new direction in Stegosaurus. Here the 

 ischium becomes shortened and flattened ; the postpubis does the same 

 and moreover applies itself closely along the ventral margin of the 

 ischium. But morphologically there is here no return to the former 

 triradiate condition of the pelvis, since the ischium has kept some 

 trace of the form wdiich it acquired in the biped phase, and the pos- 

 terior branch of the pelvis is no longer formed by the ischium alone 

 but by the ischio-postpubic complex. While evolving in the new 

 direction the postpubis has thus changed in function (Dollo, 1. c. p. 

 447). 



We find an illustration of the first alternative of the third law in 

 the evolution of the arms in the Octopods. These animals in adapt- 

 ing themselves to bottom sea life have lost a pair of arms (the tentac- 

 ular arms) possessed by their immediate ancestors the heteropod de- 

 capods. They have thus become isopods again (excepting the pecu- 

 liar case of the Argonaute, and the hectocotylisation of one of its 

 arms) like their distant ancestors the Belemnoteuthids (isopod de- 

 capods) without having been able to regain the same number of arms 

 (see Dollo, 17, pp. 115-116). x 



The best illustration for the second alternative of the third law is 

 the foot of Dendrolagus, an arboreal Kangaroo. The structure of 

 the foot in the saltatorial Macropodidae — the predominance of the 

 fourth toe, the syndactylism of the second and third, the reduction of 

 the fifth and the complete disappearance of the great toe — shows us 

 that their immediate ancestors were arboreal. In Dendrolagus, a 

 Macropodid which has again become arboreal, the opposable great 

 toe, completely atrophied in its immediate ancestors the terrestrial 

 kangaroos, was not able to reappear. But the unreduced parts of 

 the foot have undergone an ascending evolution in a new direction. 

 While the metatarsals and the phalanges have diminished in length, 

 the phalanges and claws have become lengthened and the claws have 

 at the same time become curved. Thus the foot of Dendrolagus has 

 not been able to return to the structure of the foot of its distant 



1 A still more convincing instance would be the secondary steganocephaly of the che- 

 lonians. This steganocephaly differs from the primary steganocephaly of the ancestral 

 steganocephalous amphibians in that the postorbital, the supnatoniporal, and the epioUc 

 do not reappear in the cranial vault when once lost (see Dollo, 19, p. 59). But the 

 secondary steganocephaly of the chclonians, although very probable (see especially the 

 recent note by G. A. Boulenger, " Sur la place des Cheloniens dans la classification " in 

 Comptes rendus, vol. 167, 1918, p. 514), is not yet beyond doubt. See D. M. S. Watson, 

 Eunotosaurus africanus, in Proc. Zool. Soc, London, 1917, p. 1011. 



