686 REPORT— 1897. 



of ligbt in many fishes. A striking instance is the lower side of flatfishes. It is 

 also known that Proteus, when exposed to the light, hecomes dark (Oshorn), and 

 that the lower side of a flounder, if exposed to the light, may become pigmented 

 (Cunningham). 



Now, since pigmentation cannot be of any selective value in dark places, the 

 disappearance of pigment cannot be attributed to natural selection ; nor can the 

 matter of economy have given selection a chance to remove the pigment. Is the 

 lack of pigment, then, a characteristic reacquired with each individual ? It is not, 

 for in a yoimg fish kept for ten months in the light the absence of pigment was 

 as marked as in the adult. 



We apparently have here an acquired characteristic, the depigmented condi- 

 tion of the chromatophores hereditarily established. 



5. The Origin of the Mainmalia. 

 By Professor Henry Fairfield Osborn. 



The Tertiary and Recent placental^ have been divided by the author into Cen- 

 eutheria and Meseutheria. 



The former include the higher types, progressive and specialised, mainly during 

 the Eocene and Oligocene periods. The latter include the lower types, persistent 

 and primitive, specialised mainly during the Mesozoic period, and with the excep- 

 tion of the Lemaroidea, Insectivora, and Ganodonta, dying out early in the Ter- 

 tiary. Among these Meseutheria are included the Creodonta, Tillodontia, Insec- 

 tivora, Lemuroidea, Condylarthra, and Amblypoda. The most distinctive feature 

 of their evolution is the retarded brain development, the inertia or persistence of 

 many primitive characters lost among the Oeneutheria, as well as the fact that they 

 appear substantially in their fully specialised form in the base of the Eocene, and 

 are thus distinctively the Mesozoic placentals. The known upper Cretaceous 

 mammals are substantially of the same Eocene Meseuthere type, and contain also 

 certain Multituberculata (which may be regarded as Prototheria) and possibly also 

 certain marsupials. 



The Lower Cretaceous or Upper Jurassic (Purbeck) Mammalia embrace also Mul- 

 tituberculates (? Prototheria), Triconodonts (Metatheria), and Insectivora primitiva 

 (Meseutheria). The latter may have given rise to the later Meseutheria, and thus 

 indirectly to the Ceneutheria, although no absolute links are as yet established 

 connecting the Ceneutheria with the Meseutheria, and the latter are even more 

 primitive than the known forms of Metatheria or Marsupialia. 



The combined characters of the three above-mentioned types of Jurassic mam- 

 mals led the author in 1891 to the conclusion that the Hypotheria or Promam- 

 malia would be found to possess a heterodont dentition, consisting of I. 4, C. 1, 

 P. 4-5, M. 8. Also that all the Mammalia, multituberculate as well as trituber- 

 culate, would be found to be originally derived from a trituberculate type of molar 

 dentition. 



In the meantime Baur has shown that Cope's Pehjcosauria, a division of the 

 Theromorpha, which Cope believed to be ancestral to the mammals, must be en- 

 tirely removed from this position. The discoveries of Seeley in the Permian of 

 South Africa (Karoo Beds) show that the Theriodontia possess most of the charac- 

 ters which we may expect to find in the ancestors of the Mammalia, mingled with 

 many distinctively reptilian characters. Among these Theriodonts the herbivorous 

 division, or Gomphodontia, presents many analogies to the Multituberculata, while 

 the carnivorous Cynodontia are similarly analogous both to the Protodonta (Os- 

 born) of the American Triassic and to the Triconodonta, or ancestral tritubercu- 

 lates, the specialised dental formula agreeing closely with that postulated for the 



