2 B. S. Lull — Dinosaurian Distribution. 



grateful to my colleagues Professors Schuchert and Barrell for 

 helpful criticisms and suggestions. 



II. Classification. 



The dinosaurs, because of their great adaptive radiation 

 throughout their long career, seem to be a very heterogeneous 

 group, so much so that Bairr (1891) emphatically denied any 

 relationship on the part of the several orders which are 

 included within the group. 



They exhibit two distinct lines of cleavage, dividing the 

 super-order into three orders, of which two, the carnivorous, 

 bipedal Theropoda and the herbivorous, quadrupedal Sauro- 

 poda, may be grouped together, in spite of great adaptive dif- 

 ferences ; while the herbivorous, bipedal or quadrupedal Ortho- 

 poda stand more aloof and show a vastly greater range of 

 intra-ordinal variation. To the first two orders collectively 

 the name Sanrischia has been given by Seeley (1888), while 

 the Orthopoda have been designated by the .corresponding title 

 of Ornithischia. 



A further differentiation of the Theropoda points to two 

 distinct races. The heavier megalosaurs, typified by Megcdosau- 

 rus of Europe, Allosaurus of the American Morrison, and 

 culminating in the huge Tyrannosaurus of the Laramie, 

 were the more conservative group, the evolution of which 

 consists mainly in an increase in size, accompanying a relative 

 diminution of the fore limbs, which were never used for loco- 

 motion, and an assumption of the prehensile function by hind 

 feet and mouth armament as in birds of prey. 



The other carnivores, the compsognathoid forms, were of 

 an aberrant nature, not increasing so markedly in size; but of 

 a more agile character, better fitted to prey upon feebler rep- 

 tiles, birds and mammals than upon other dinosaurs. Typical 

 members of this sub-order are Compsognathus from the litho- 

 graphic stone of Bavaria and Ornitholestes of the American 

 Morrison beds. 



The Sauropoda appear and disappear with startling sudden- 

 ness in the northern hemisphere, though lingering in the 

 southern until the close of the Mesozoic. During this time 

 they undergo but little evolution, and that mainly in the line 

 of a marvelous adaptation for lightness coupled with great 

 strength, especially in the elongated vertebral column. Per- 

 haps the best-known representatives of this group are Ajpato- 

 saurus {Brontomurus), a huge, nnwieldly beast, and Diplo- 

 docus, a lighter, more slender form ; both from the Morrison 

 of Wyoming and Colorado. It is a significant fact that the 

 most generalized {Ilajplocantliosaurus) and the most specialized 



