JURASSIC FAUNA. 161 



birds have as yet been discovered in deposits of Triassic age.* But 

 it is not impossible, or even improbable, that some of the smaller 

 foot-prints that are scattered about the larger ones, and which in 

 some instances are disposed in a single series one in advance of the 

 other, indicating a method of progression adopted by certain wad- 

 ing birds, may actually be of an ornithic nature. However this 

 may be, it is certainly a significant fact bearing upon the doctrine 

 of evolution, that no unequivocal traces of birds have thus far been 

 discovered in deposits antedating those which contain the remains 

 of reptiles, which in their several characters most approximate the 

 birds, and in reality eSsct a transition to them. The progressive 

 evolution of advanced or most specialised types is here clearly 

 indicated. 



The Mammalia, the highest class of vertebrates, appear for the 

 first time in the deposits of this age. They are indicated by the 

 teeth and fragments of jaws pertaining to two or three genera, 

 Dromatherium, Microlestes, and Hypsiprymnopsis, forms, as nearly 

 as can be determined, belonging to the low type of the Marsupialia, 

 and, probably, more or less closely allied to the modern banded 

 ant-eater (Myrmecobius) and kangaroo-rats (Hypsiprymnus) of Aus- 

 tralia. 



Jurassic Fauna. — The life-history of the Jurassic period, while 

 combining certain prominent features not hitherto recognised, pre- 

 sents to us primarily an expansion of those characters with which 

 we have just become acquainted. The remarkable group of the 

 dinosaurian reptiles, whose development in the Triassic period had 

 but barely passed beyond its own beginnings, acquires here re- 

 newed importance, apart from the mere matter of numbers, from the 

 circumstance of the gigantic and diverse forms which it includes. 

 Four distinct types of this order are recognised, all of which had 

 representatives in the Jurassic period: 1. The Sauropoda, lizard- 

 footed vegetable-eaters, in which the anterior and posterior pairs 

 of limbs were of nearly equal length, and whose progression was 

 effected on all fours. Among the more important genera of this 

 period belonging to the group are Atlantosaurus, Brontosaurus, 

 Morosaurus, and Cetiosaurus, the first, from the deposits of the 



* Since the .ibove wns written announcement lins been made of the dis- 

 covery of the skeletal remains of a traek-mnlvins dinosaur of the Connecticut 

 Valley. Trans., New York Ap. Soiapoe.s, Oct. 26, 1S85. 



