163 GEOLOGICAL DISTEIBUTIOK. 



Eocky Mountains, measuring from eighty to one hundred feet in 

 length — the largest land animal with which we are acquainted. 



3. The Stegpsauria (Stegosaurus, Scelidosaurus), armoured vegeta- 

 ble-feeding dinosaurs, some of them of gigantic frame, whose pro- 

 gression, owing to the feeble development of the anterior pair of 

 limbs, appears to have been in great part effected by means of the 

 hinder extremities alone. In Scelidosaurus Harrisoni, from the 

 Lias of Dorsetshire, the hind foot measured three feet and a half in 

 length. 3. The Ornithopoda, bird-footed herbivores, with a very 

 unequal development of the anterior and posterior appendages, 

 the latter closely approximating the structure found in birds. 

 There can be but little question as to the habitually erect posture 

 assumed by such forms as Camptonotus, Laosaurus, and Iguanodon. 

 Iguanodon Mantelli, a Cretaceous species, measured about thirty 

 feet in length from the tip of the nose to the extremity of the tail. 

 No member of this genus is known from the American deposits. 



4. The Theropoda, carnivore forms, whose progression was largely 

 erect, and assisted in many cases, probably, by the greatly developed 

 tail acting as a fulcrum, in the manner of that organ among the 

 kangaroos. This type, which is almost alone represented in the 

 Triassic deposits, includes the' most formidable members of the 

 order — Megalosaurus, Allosaurus, Dakosaunis, the former appar- 

 ently attaining a length of fifty feet. The genus Compsogna- 

 thus, represented by a single species (C. longipes) from the Upper 

 Oolite, possesses probably the greatest number of avian characters 

 of the entire order, and is considered to stand in the direct line of 

 the descent of birds. 



Monsters parallel to those of the land- surface inhabited the 

 oceanic waters, such as the finned Plesiosaurus and Pliosauras, and 

 the not distantly removed Ichthyosaurus and its American tooth- 

 less ally, Sauranodon. The geographical distribution of Ichthyo- 

 saurus is a very remarkable one. While apparently the genus is 

 completely wanting in the deposits of the New World, its range in 

 the Eastern Hemisphere embraced very nearly its whole north and 

 south extent, from Spitzbergen (Ichthyosaurus polaris, Triassic) to 

 Australia (I. australis, Cretaceous). Its greatest developmetft ap- 

 pears to have been in the early part of this period (Lias). From 

 the discovery of fragmentary parts of young individuals within the 

 bodies of more fully de^jeloped ones, it has been conjectured that 



