THE TRIASSIC PERIOD. 45 



(Dromotherium and Microconodon) and one in Europe (Microlestes) . 

 This early appearance of the mammals, while yet the reptiles were 

 strongly ascendant, doubtless indicates a very early ancestry, suggesting 

 that perhaps the mammalian divergence began while yet their ancestors 

 were stegocephalians, as some believe, or in the very early stages of 

 the reptilian evolution in connection with the theromorphian develop- 

 ment, as others believe. In view of the mammalian dominance of 

 the recent ages, it is not a little instructive to note that the non- 

 placentals developed very slowly and feebly in America and Europe 

 during the whole Mesozoic era. Question has even been raised whether 

 the placental mammals are the descendants of these Mesozoic non- 

 placentals, with the suggestion that perhaps they had an independent 

 and equally early origin, a question on which future studies in Africa, 

 where the theromorphs had their strongest early development, is 

 likely to throw light. 



The reptiles go down to sea. — Both wings of the reptilian horde 

 sent delegations to sea before the close of the period, the thalatto- 

 saurians and ichthyosaurians representing the more declaredly reptilian 

 line, and the sauropterygians (plesiosaurians) representing the mam- 

 malian branch. This similarity of movement and of adaptation has 

 associated the ichthyosaurs and plesiosaurs in geological thought, 

 though they are not close allies biologically. It is not difficult to 

 find good reasons for this movement to the sea. Besides the inevi- 

 table tendency of every masterful race to invade all accessible realms, 

 the renewed extension of the sea that set in during the Triassic period 

 and became pronounced before its close, especially invited this; for 

 the shallow waters, creeping out upon the land, with their now pro- 

 lific life, set tempting morsels before the voracious reptiles, on the one 

 hand, while on the other, the reduction of the land area and the re- 

 striction of their feeding-grounds, intensified by their own multipli- 

 cation, forced a resort to the sea. 



The sauropterygians seem to have been the leaders in this move- 

 ment and to have become almost at once lords of the sea, and to have 

 preyed upon the previous rulers, the fishes. The nothosaurs were 

 the earlier and more primitive type of the sauropterygians and reached 

 their climax and closed their career within the period; but true plesio- 

 saurs were present. The accompanying restoration of the skeleton of 

 Lariosauras , a genus confined to the Trias, illustrates by its well- 



