JUKASSIC ANIMALS. 449 



cially the earliest or Triassic, decidedly reptilian in some of their char- 

 acters. Of these we mention only two. 



1. Teeth. — A glance at Fig. 711 a, in comparison with figure of The- 

 riodont (Fig. 624 Z>), on page 420, shows that the teeth of some of the 



4b 



b c d e f 



Fig. 711.— A Series of Molars or Triassic and Jurassic Mammals: a, Dromatherium: b, Micro- 

 nodon; c, Amphiiestes; cl, Phascolotherium; e, Tncouodon; /, Splalacothenum Rafter Osboni). 



earliest mammals differed little from those of Theriodont reptiles ; in 

 which, as already explained, the tuberculation of the molars character- 

 istic of mammals had already commenced. In the subsequent course 

 of evolution the subordinate cusps of Fig. 711, a, 6, c, d, e, were 

 shifted outward in the upper jaw and inward in the lower jaw, so that 

 the cusps interlocked. This forms the tritubercular molar of Cope 

 (Fig. 711/), so common in Mesozoic animals. From this simple gener- 

 alized type were afterward developed the more complex molars of the 

 specialized animals of the Tertiary and present time. 



2. Reproduction. — There seems to be no doubt that many of these 

 animals were marsupials, although more generalized than any existing 

 marsupials. Now, marsupials in their reproduction approach reptiles. 

 In typical mammals the embryo is attached to the mother by a pla- 

 centa, so that the whole embryonic development is within the uterus ; 

 in marsupials, on the contrary, there is no placental attachment, and 

 therefore the intra-uterine development is very short and imperfect, 

 and is, in fact, completed outside the uterus in the pouch (mar- 

 supium). In true mammals the whole embryonic development is 

 within (gestation), and the young are born in perfect condition. They 

 are young-bearers {viviparous). In birds and reptiles — egg-bearers 

 {oviparous) — the whole of the development takes place without (incu- 

 bation). In marsupials the development is partly within hut mostly 

 ivithout. These, therefore, may be called semi-oviparous, or reptilian 

 mammals. 



Thus the class of mammals has been divided into two sub-classes — 

 placentals or true mammals, and non-placentals or reptilian mam- 

 mals. The latter includes the marsupials, semi-oviparous, and the 

 monotremes (Ornithorhynchus, Echidna, etc.), which are true egg-lay- 

 ers (oviparous), like birds and reptiles. The non-placentals, with the 

 exception of a few opossums in America, are wholly confined noiv to 

 the Australian region. In Jurassic times they roamed in great num- 

 bers all over Europe and America. 



Origin of Mammals.— In Theriodonts and Theromorphs we see 

 29 



