THE EVOLUTION OF MAMMALIA. 521 



Stonesfield with teeth of placental type is Stereoynathus. The affinities 

 of the numerous mammals from the Purbeck beds, Spalacotherium, Tri- 

 conodon, Galestes, Plagiaulax t &c. t are apparently marsupial. 1 There are 



Fig. 147. Phascolotherium. Lower jaw enlarged. 



no herbivorous marsupials in the Tertiary rocks ; and Professor Gaudry 2 

 urges that this group would be at a disadvantage in the struggle for ex- 

 istence, as compared with higher herbivorous mammals, from the way 

 in which the movements of the parent are controlled by the helpless 

 condition of the young, and that this disadvantage might have led to 

 their extermination in the European Tertiary area. Carnivorous 

 marsupials, which so closely resemble the existing opossums that it is 

 not easy to separate them by generic characters, are found in the 

 older Tertiaries of Paris, Auvergne, and Vaucluse. These Carnivorous 

 marsupials of Insectivorous type are termed by Professor Cope Creo- 

 donta. 3 Hycenodon and Pterodon, somewhat intermediate between the 

 placental and implacental types in their dentition, are inferred by 

 Gaudry to have been marsupials from the form of the axis vertebra. 

 Professor Huxley regards them as connecting the Garni vora and In- 

 sectivora. This intermediate character is found in other genera, such 

 as Palceonictes and Cynohamodon, from the Phosphorites of Quercy. 

 Ardocyon is an omnivorous mammal, resembling the marsupial bears. 

 This blending of characters in Tertiary genera is intelligible on the 

 hypothesis that implacental mammals of the Secondary period of geo- 

 logical time become ultimately developed into the insectivorous placen- 

 tal type ; and in this way Professor Gaudry interprets the resemblances 

 to marsupials seen in some fossil carnivora and in living lemurs. 



There is no fossil evidence as to the parentage of Cetacea. Palceo- 

 cetus Sedffwicki 4 is recorded as a fossil from the Secondary rocks on 

 the evidence of its mineral condition, which is that of fossils from the 

 Secondary clays. Zeuglodon occurs in the Barton clay, Squalodon in 

 the Miocene rocks and crag, Balenoptera dates from the Headon beds ; 5 

 Halitherium, from the Crag and Middle Tertiary, is intermediate 

 between the Dugong and Lamantin ; Pugmeodon is intermediate 

 between the Lamantin and Halitherium. 



1 R. Owen : " Purbeck Mammalia : " Palseontographical Society. 



2 See Gaudry : " Enchainments du Monde Animal," one of the most important 

 modern contributions to Palaeontology, in which most of the following facts con- 

 cerning mammals may be found. 3 "American Naturalist," 1884. 



4 Geol. Mag., Feb. 1865. p. 54. & Q. J. G. S., vol. xxxvii. 



