466 



MESOZOIC ERA— AGE OF REPTILES. 



were well protected with broad plates, some of which have been found 

 three feet in diameter, and armed with sharp spines two feet long. 

 These latter were placed on each side of the powerful tail, near the 



Fig. 751. — Stegosaurus stenops, x J (after Marsh). 



end, and must have been very formidable weapons of offense. The dis- 

 parity in size of hind and fore limbs (Fig. 749), greater than in any 

 known Dinosaur, shows that they walked habitually on the hind-legs, 

 like the Ornithopoda. Like them, also, they probably had three to 



four toes on the 

 hind - feet. The 

 brains of all Juras- 

 sic Dinosaurs were 

 very small in com- 

 parison with living 

 reptiles, but this 

 was especially true 

 of Stegosaurs. To 

 make up for this 

 deficiency they had 

 an enormous enlargement of the spinal cord in the sacral region. This 

 sacral brain — if we may so call it — was ten times bigger than the cran- 

 ial brain (Fig. 750). It was necessary in order to work the powerful 

 hind -legs and tail. 



Ichthyosaurs. — Besides the Dinosaurs, Marsh describes from the 

 same formation (Jurassic), but from a lower horizon, an Ichthyosau- 

 rian, but differing entirely from the Ichthyosaurus of the European 

 Jurassic in being toothless. On this account he calls the genus Bap- 

 tanodon. This reptile had six digits in both fore and hind feet — a 

 new and most remarkable feature (Fig. 752). 



Birds. — In 1881 Marsh discovered in the same beds, the Atlanto- 

 saur beds of Wyoming, a Jurassic bird (Laopteryx), the only one yet 

 known in America. It was undoubtedly a reptilian bird, probably with 



UO °Ooo- T 



Fig. 752.— Left hind paddle of Baptanodon discus (after Marsh), seen 

 from below. One eighth natural size: f. femur; t, tibia; i, interme- 

 dium; /', fibula; /, first digit; V, fifth digit. 



