THE JURASSIC PERIOD 719 



ranging from 13 to 76. The neck appears not to have been so 

 flexible as familiar illustrations have represented it, nor were the 

 jaws separable and extensible as in the case of snakes. This implies 

 either that they lived on small prey, or tore their food to pieces 

 before swallowing. They were doubtless formidable foes of the 

 smaller sea animals, but probably not of the larger. Like the 

 ichthyosaurs, they were without scales. They ranged from 8 to 

 40 or more feet in length. 



Marine crocodilians made their appearance in the later part of 

 the period. They had undergone a remarkable adaptation to the 

 sea (Fig. 491). They were fish-like in appearance, their skins were 

 bare, and their tails terminated in a large fin like that of the ich- 

 thyosaurs. The fore limbs were short and paddle-like. The hind 

 limbs were modified but slightly from the land type, perhaps due 

 to the recurring necessity of visiting the shores for depositing and 

 hatching their eggs. 



Marine turtles, so characteristic of the Cretaceous, had not yet 

 appeared. 



The Land Life 



Vegetation. The land vegetation of the Jurassic was little more 

 than a continuation and expansion of that of the late Triassic, with 

 slow progress toward living types. Cycadeans, conifers, ferns, and 

 equiseta were the leading plants, slightly more modernized than 

 their Triassic ancestors, but not changed radically. 1 The cycadeans 

 (Bennettitales and Cycadales) were perhaps the most distinctive 

 forms, though the conifers showed the more notable modernization. 

 They embraced yews, cypresses, arborvitas, and pines, all of which 

 had a somewhat modern aspect, though all the species are extinct. 

 The ginkgos also played a somewhat important role. 



An interesting feature of the European record is the rather 

 frequent occurrence of land plants in marine beds, which implies 

 that many trunks, twigs, leaves, and fruits, were floated out to sea, 

 and that the landward edges of the marine deposits have escaped 



1 For a comprehensive paper on the Jurassic plants of the United States, 

 with descriptions and illustrations by Lester F. Ward, see 20th Ann. Kept., 

 U. S. Geol. Surv., 1898-99, pp. 334-430. 



