270 ANNALS NEW YORK ACADEMY OF SCIENCES 



MONOTREMATA 



The monotremes are the lowest group of mammals, far removed struc- 

 turally from any others. Their connection with the main stock must 

 date back to the end of the Paleozoic era. Xothing is known of tlieir 

 evolutionary history. The Multituberculata of the Mesozoic and Basal 

 Eocene are regarded by Broom as ancestral to them, but this view is not 

 supported by additional evidence since obtained. Xe7iotherium^° of the 

 North American Oligocene, referred by its describer to the monotremes, 

 is an Insectivore related to the Chrysochlorida; ; Scotwops^^ of the South 

 American Tertiary is an Armadillo,^- and other genera referred by 

 Ameghino to the Monotremes probably also pertain to other groups. 

 We find them to-day limited to the Australian region, and surviving 

 even there only by virtue of unusual specializations of habit; Echidna 

 protected by its coat of spines, Ornithorhynchus by its amphibious habitat, 

 both genera burrowing and nocturnal. Presumably, these genera repre- 

 sent the last relic of the early Mesozoic dispersal movements of tlie 

 Mammalia. 



SUMMARY OF THE EVIDENCE FROM DISPERSAL OF LAND MAMMALS 



The foregoing review of the several groups of land manmials shows 

 that the more recently evolved and dominant races of Mammalia are to- 

 day mainly Holarctic, and many of them have not yet reached the more 

 peripheral regions ; that the ancestry of all these dominant races has been 

 found in the Holarctic Tertiary formations, sometimes in Europe, some- 

 times North America, more generally a series in each country of equiva- 

 lent approximately ancestral stages. Where the geological record is ade- 

 quate, these races are shown to be newcomers in the peripheral continents 

 which they have invaded, and any ancestral series is absent. Their repre- 

 sentatives in the peripheral continents are to a varying degree primitive 

 and allied to earlier stages in the evolution of the race as represented in 

 the Tertiary record of Holarctica, but they have specialized more or less 

 along parallel or divergent lines from the direct line of descent of the 

 northern representatives. 



When the parallel series in Europe and North America are sufficiently 

 complete they are seen to be not parallel phyla of independent local evo- 

 lution, but periodically recruited by more progressive new stages, appar- 



90 Earl Douglass, 1905. (The name is preoccupied by A' en other in m Ameghino, 1904, 

 a genus of typotheres.) 

 Bi Fl. Ameghino, 1887. 

 "" W. B. Scott : Rep. Prin. Exp. Patag., vol. 5, p. 12. 1903. 



