694 GEOLOGY 



Trias, and most of them were not especially large. Their general 

 form is indicated by the partially restored skeleton shown in I-'i.u. 

 471. The strong development of the hinder parts, the relative 

 weakness of the fore limbs, and the kangaroo-like attitude, are 

 the most obvious features. The bones of these upright-walking 

 forms were hollow, and certain other structural features resemble 

 those of birds. The reduction of the functional toes of the hind 

 feet to four, with one of them much shorter than the others, caused 

 their three-toed tracks to be mistaken for those of birds, until 

 recently. Even in the Triassic period, the dinosaurs ranged widely, 

 living in the Rocky Mountains, along the Atlantic coast from 

 Carolina to Prince Edward Island, in western Europe, India, and 

 South Africa. 



Before the close of the period both branches of the reptilian tribe 

 sent delegations to sea, the one represented by the ichthyosaurs 

 (Fig. 489) and the other by the plesiosaurs. It is not difficult to 

 find good reasons for this sea- ward movement. Besides the ten- 

 dency of every masterful race to invade all accessible realms, the 

 notable extension of the sea before the close of the period invited 

 the adaptation of land animals to an aqueous habitat, for the shallow 

 waters with their prolific life, creeping in upon the land, set tempting 

 morsels before the voracious reptiles, while the reduction of the 

 land areas and their terrestrial feeding-grounds, intensified by their 

 own multiplication, encouraged a movement to the sea. 



The advent of mammals. Of peculiar interest is the appearance 

 of early forms of non-placental mammals. They were small, and 

 so primitive in type that it is not altogether certain that they were 

 mammals; but they are commonly regarded as such, with kinship 

 to the marsupials. Their appearance while reptiles were yet dom- 

 inant suggests that mammals diverged from the primitive stock 

 much earlier. In view of the mammalian dominance of later times, 

 it is note-worthy that the non-placentals developed but slowly and 

 feebly during the Mesozoic era. It is an open question whether 

 placental mammals are the descendants of the Mesozoic non-pla- 

 centals, or whether they had an independent origin. 



