OBSERVATIONS ON THE GEOLOGICAL DEVELOPMENT, 

 ORIGIN AND DISTRIBUTION OF MAMMALS.^ 



Up to the present time no mammalian remains have been reported from 

 pre-Triassic deposits. Mammals are even somewhat rare in the Triassic, 

 iDut are nevertheless represented by two distinctly different types, namely, the 

 Allotheria and the Dromatheriidae, which remind one of insectivores on the 

 one hand, and polyprotodont marsupials on the other. The Allothma may 

 be divided into the genera Microlestes and Triglyplms from the Ehaetic Bonebed 

 of Wiirtemberg, and the genus Tritylodon (Theriodesmus) from the Karroo strata 

 of South Africa. These forms indicate that the Allotheria, which became 

 extinct in the Eocene, were already very widely distributed at this time. 

 The latter type {Dromatheriidae) is represented by the small mandibles of 

 the genera Microconodon and Dromatherium from the Upper Triassic of North 

 Carolina, also possibly by two tiny mandibles which Avere found a few years 

 ago in the Triassic of Cape Colony and have been described as Tribolodoii 

 and Karoomys. It is very questionable whether these specimens belong to 

 the Mammalia. 



The mammalian remains from the Jiirassic are decidedly more numerous, 

 for the Allotheria as well as the polyprotodont Marsupialia during this time 

 develop a considerable variety of forms. In Europe no new forms have lately 

 been discovei'ed from the long known localities of the Great Oolite of Stones- 

 field, and the " Dirtbed " of Purbeck, England ; instead the Atlantosaurus beds 

 of Wyoming and Colorado have yielded a number of mammals, which in part 

 are closely related to well-known forms from England and in part even belong 

 to the same genera as these. The Allotheria are represented in England by 

 the genus Plagiaulax, and in North America by the genera Allodnn and 

 Ctenacodon. The remaining mammals of the Jurassic have a secodont dentition, 

 and on account of their similarity to the recent genus Myrmecohius were 

 classified b}^ Owen with the polypi'otodont marsupials. Osborn showed later 

 that several of these forms are very like certain insectivores in dental and 

 mandibular structure, and that the dental formula of one genus, Peramus, is 

 absolutely identical with that of the primitive placentals. Other forms, 



^ Ameghino, F., Les Formatious sedimentaires de Patagonie. Anal, del Museo de Buenos 

 Aires, vol. xv., 1906. — Lydekker, R., A Geographical History of Mammals. Cambridge, 1896. — 

 Marsh, 0. C, Introduction and Succession of Life in America. Amer. Assoc, for Advancement 

 of Sci., Nashville, 1877. — Matthew, W. D., Classification of the Freshwater Tertiary of the 

 West. Bull. Amer. Mus. Nat. Hist., New York, 1899. — Cenozoic Mammal Horizons of Western 

 North America. Bull. U.S. Geolog. Survey, No. 361, 1909.— Os^wn, H. F., The Rise of the 

 Mammalia in North America. Amer. Assoc, for Advancement of Sci., Madison, 1893. — Cor- 

 relations between Tertiary Mammal Horizons of Europe and America. Ann, N.Y. Acad. Sci., 

 vol. xiii., 1900.- — Stehliyi, H. G., Die Saugetiere des schweizerischen Eocans. Abh. Schweiz. 

 Paliiont. Ges. 1903-1910. — Mammiferes eocenes et oligocenes du bassin de Paris. Bull. Soc. 

 Geol. de France, 1909. 



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