JURASSIC VERTEBRATE FOSSILS. 481 



phases of ancienl life as it developed from age to age in the progress of the 

 world. They thus afford interesting data as to the higher forms of life in 

 ages still more remote, and, more important yet, indicate' the lines of descent 

 which have continued on to the present. 



During Mesozoic time the vertebrate life consisted mainly of fishes, 

 amphibians, and reptiles, although birds and mammals wen- also represented. 

 of all these the reptiles were the dominant forms, and the title "Reptilian 



Age" appropriately defines the whole period. In this class the dinosaurs 

 form in many respects the most important group, and the representatives 

 that left their remains in the neighborhood of Denver are not surpassed in 

 interest by those found in any other part of the world. 



To make clear the descriptions that follow, it may he well to state here 

 that dinosaurs are now divided into several distinct orders: The Theropoda, 

 or carnivorous, bipedal forms; the Sauropoda, including huge herbivorous, 

 quadrupedal forms; and the Predentata, consisting of herbivorous reptile- of 

 several very different suborders, among them the Stegosauria, or plated 

 lizards, the ( !eratopsia, or horned forms, and the ( >rnithopoda, or bird-footed 

 reptiles, which in many respects resemble birds. A full account of these 

 remarkable reptiles will he found in an illustrated memoir by the writer. 

 entitled "The Dinosaurs of North America," and published in Part I of the 

 Sixteenth Annual Report of the United States Geological Survey. With 

 these various dinosaurs lived other reptiles, especially crocodiles and turtles, 

 and numerous fishes. A few birds and some diminutive mammals likewise 

 then existed in the same region. 



Kni'TILIA. 



HALLOPUS. 



Near the has<- of the Jurassic strata not far from Canyon, in the hori- 

 zon already defined as the Hallopus beds, remains of a small carnivorous 

 dinosaur have been found, which are worthy of special description. This 

 diminutive reptile, which has been named by the writer Hallopus victor, was 

 but little larger than a domestic fowl, although of much more slender pro- 

 portions. The greater part of the skeleton is preserved, and this shows 

 that the bones were bird-like anil hollow, possibly pneumatic, and in their 



MOX XXVII 31. 



