D PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



a distinct order, the Hippoidea [op. cit). Zittel also does not recognize the 

 group as of even subordinal rank, but includes the families among the peris- 

 sodactyls ('91, 263, 267), an example which is followed by Gaudry and appar- 

 ently also by Roth ('03). Lydekker, on the other hand, holds a very different 

 view and believes that the Litopterna, the Toxodontiaand the Astrapotheria 

 "have originated from a common ancestral stock, though apparently before 

 the perissodactyles were differentiated from an earlier group known as the 

 Condylarthra " ('96, 77). This represents very nearly the conclusion 

 which I have reached, that the Litopterna are more nearly related to the 

 Toxodontia than to the Perissodactyla, and that the striking resemblances 

 to the latter are largely due to parallelism of development, but in part 

 also to the retention of certain primitive characters once common to all 

 ungulates, or even to all mammals. 



The significant likenesses between the Litopterna and the Perissodactyla 

 are, to a very great extent, conditioned by the fact that in both groups the 

 feet are constructed upon a plan of mesaxonic symmetry, while such 

 structures as are unaffected by this symmetry are, for the most part, funda- 

 mentally different in the two orders, and are much more primitive in all 

 of the Litopterna than in the most ancient known perissodactyls. Indeed, 

 one highly significant result of a comparison of the two groups is that 

 their earlier representatives are no more alike than their later members. 

 In other words, so far as their history can be traced back, the two orders 

 show no signs of converging to a common ancestry. 



The resemblances between the Litopterna and the Perissodactyla are 

 not so much between the two groups, as wholes, as between the horses 

 and the Proterotheriidae, which are certainly very remarkable, and yet 

 even here, the fundamental dissimilarity of plan is none the less obvious, 

 as revealed in all parts of the skeleton, but especially in the carpus and 

 tarsus. In addition to the mesaxonic symmetry of the feet and the lophodont 

 molar plan, the only other point of special resemblance to the perisso- 

 dactyls is the third trochanter of the femur. This, however, is a feature 

 common to nearly, if not quite, all early mammalian groups, and there- 

 fore is of no great weight in deciding questions of relationship. 



The more significant differences between the Litopterna and the Perisso- 

 dactyla are expressed in the following table. 



