AND AVES OF NORTH AMERICA. 89 



posterior to its position in ordinary reptilia. This, taken in connection with the anterior 

 position of the support of the body — the femur, rendered the erect progress of the Dino- 

 sauria possible. 



Another approximation to the birds will probably be found in the sternum and 

 coracoids. These elements are but little known, and that imperfectly ; the best example 

 has been furnished by the great Teratosaurus suevicus Mey. Here, according to Plienin- 

 ger, the elements corresponding to the xiphisternum of Lacertilia is a large thin shield- 

 like bone, of elongate form. The coracoids are narrow, prismatic bones, and abut against 

 the anterior angles of the xiphisternum ; being entirely different from the broad flat 

 element of the Lacertilia and other orders, which are usually extensively in contact with 

 each other or. with the xiphisternum. 



We have, however, among Dinosauria, as among Quadrumanous Mammalia, a series of 

 forms, from those constantly assuming the prone Lacertian position, to those that walked 

 exclusively erect like birds. Perhaps the most Lacertilian form known is the genus 

 Scelidosaurus of Owen : the greater equality in length of the limbs, and the numerous 

 toes, as well as lacertilian dentition assign it to this place. Then we find forms like 

 Iguanodon and Hadrosaurus, the most gigantic of land animals, where a semi-erect atti- 

 tude was the natural one, as they like the Megatherium and Megalonyx, lived on vegeta- 

 ble food, and were necessitated to raise themselves on their hinder limbs to reach it. Here 

 the bird-like type is approached, in the reduction of the metatarsi to three, and the great 

 antero-posterior extent of the ilium. In the genus Laelaps the position was probablv 

 quite erect, and additional resemblances to the ornithic type are adapted to large animals 

 no longer requiring a vegetable diet, but procuring their living food by activity and 

 strength. They are accordingly organized so as to be entirely independent of extraneous 

 support, and furnished with great powers either of running or leaping. 



Intermediate between this extreme, and the type of Ignanodon, comes a large carniv- 

 orous genus, the Megalosaurus of Buckhmd, the representative of types like Laelaps, in 

 the old world. In its longer fore limbs it differs from the most bird-like forms. A car- 

 nivorous type only known from teeth, is Aublysodon Leidy ; it is American. 



The other herbivorous species, of less size than Iguanodon, which was furnished with a 

 dorsal series of dermal bones, is the Hylaeosaurus armatus Mantell, found in the Wealden 

 of England ; while an allied form which was covered with long massive dermal spines, 

 has recently been discovered in the same formation in the Isle of Wight, and referred to 

 the genus Polyacanthus Owen. 



The sizes of the best known species of these genera are as follows : 



AMERI. PHILOSO. SOC. — VOL. XIV. 23 



