SAUROPTERYGIA 95 



Cretaceous times. At no time do they appear to have been especially 

 numerous, nor does it seem probable that they were ever a domi- 

 nant type of marine vertebrate life, though their remains occur 

 everywhere that marine deposits of the Jura and Cretaceous are 

 known. Indeed, it may be said with almost certainty that rocks of 

 these ages and of that character everywhere in the world contain 

 fossil plesiosaurs. Their bones have been made known from 

 Europe, Asia, Africa, AustraUa, and North and South America. 

 From North America thirty or more species have been described 

 from New Jersey, Alabama, Mississippi, Texas, Arkansas, Kansas, 

 Nebraska, Colorado, New Mexico, Wyoming, North and South 

 Dakota, California, etc. 



The cause of their final extinction no one knows, nor can we 

 conjecture much about it with assurance. That climatic conditions 

 became unfavorable for them is highly improbable, considering 

 their cosmopolitan habits; they were not discriminating in their 

 environments. After successfully withstanding their fiercest foes, 

 the ichthyosaurs, crocodiles, and mosasaurs, and large carnivorous 

 fishes, it does not seem probable that they would succumb to lesser 

 enemies, though it may be that they were finally attacked success- 

 fully, not in the fulness of their strength as adults, but while young, 

 by more insidious enemies. More probably after their long life of 

 millions of years they had grown old, as everything grows old, and 

 had become so fixed and unplastic in their structure and habits 

 that even slight causes were at last their undoing. When we shall 

 have bridged over that still imperfectly known transition period 

 between the great Age of Reptiles and the greater Age of Mammals 

 we shall have learned more definitely some of the causes of the 

 extraordinary revolution in vertebrate life that then occurred. 

 The plesiosaurs went out with nearly all of their kind, the mosa- 

 saurs, the pterodactyls, the dinosaurs; and, so far as we now know,, 

 their places in the sea, land, and air were not immediately taken 

 by any other creatures. 



NOTHOSAURIA 



A few years after the discovery of the plesiosaurs by Conybeare, 

 the remains of animals of alUed kinds were found in the Triassic 

 rocks of Bavaria. At first they were supposed to be those of true 



