H. S. Lull — Dinosaurian Distribution. 11 



tion among the Ortliopoda, retained the cursorial character of 

 their ancestry, relying evidently upon celerity and speed rather 

 than upon weapons or armor for defence against their san- 

 guinary foes. The ideal of this type of dinosaur was perhaps 

 Laosaurus of North American Morrison and its old world 

 representative Hypsilophodon of the English Wealden. Later 

 Traehodon, and probably its ancestor Olaosaurus, the remains 

 of which a,re found repeatedly in marine rocks, became in 

 their turn semi- aquatic, possibly in their search for food 

 because of competition with the great armored forms. They 

 did not, however, rely on increasing bulk for immunity against 

 attack as did the Sauropoda ; but, by means of a powerful, 

 laterally compressed swimming tail may have been as active as 

 crocodiles in the water while still retaining a means of com- 

 paratively rapid locomotion on land. The defencelessness of 

 these creatures, so far as armor is concerned, has been beauti- 

 fully shown in the '' mummified " specimen of Trachodon, 

 discovered in 1908 by C. H. Sternberg, in Converse County, 

 Wyoming, and now preserved in the American Museum of 

 Natural History. Professor Osborn (1909, pp. 793-795) says 

 of it : " The first and most surprising impression is that the 

 epidermis is extremely thin, and that the markings are exces- 

 sively fine and delicate for an animal of such large dimensions. 

 There is no evidence in any part of the epidermis either of 

 coarse tubercles or of overlapping scales. In all parts of the 

 body observed it is entirely composed of scales of two kinds : 

 (1) larger pavement or non-imbricating scales, (2) smaller 

 tubercular scales." Osborn speaks not only of the "vigorous 

 use of the tail as a balancing, and perhaps partly as a swim- 

 ming organ," but also tells us that the " man us is completely 

 encased in the integument, and was thus web-footed." Evi- 

 dence for aquatic or semi-aquatic life. 



The armored dinosaurs make their first appearance in Sceli- 

 dosaurws of the English Lias, the possible ancestor of all of 

 the subsequent mailed types. The earliest forms were proba- 

 bly bipedal, but, as time went on, and the armor increased in 

 bulk and weight, we find these dinosaurs becoming secondarily 

 quadrupedal (Dollo 1905), losing all celerity of movement and 

 becoming sluggish, slow moving, living citadels of well-nigh 

 impregnable character. In habitat they were doubtless terres- 

 trial, as in the case of the earlier Ornithopoda with which their 

 remains are found associated. A curious differentiation of 

 armored dinosaurs occurred, correlated with a marked differ- 

 ence in the mode of defence, in that the more aggressive, men- 

 tally alert Ceratopsia used the head both for offence and 

 defence, while the stegosaurs seem to have used the tail. Ste- 

 gosaurus proper, which developed to an extreme this method 



