230 A CENTUEY OF SCIENCE 



were made the subjects of his next researches, after 

 which he described the peccaries of Ohio, and later, in a 

 much larger and most important work, the Cretaceous 

 reptiles of the United States (1865). Most of the fossils 

 discussed in this last work are from the New Jersey Cre- 

 taceous marls and of them the most notable was the 

 herbivorous dinosaur Hadrosaurus, the structure and 

 habits of which, together with its affinities with the Old 

 World iguanodons, Leidy described in detail. From 

 Leidy 's descriptions and with his aid, Waterhouse Haw- 

 kins was enabled to restore a replica of the skeleton in a 

 remarkably efficient way. This restoration for a long 

 time graced the museum of the Philadelphia Academy of 

 Natural Sciences and there was a plaster replica of it in 

 the United States National Museum. These, together 

 with plaster replicas of Iguanodon from the Eoyal Col- 

 lege of Surgeons in London, gave to Americans their first 

 real conceptions of members of this most remarkable 

 group. The associated fossils from the New Jersey 

 marls were chiefly crocodiles and turtles. 



From 1853 to 1866 F. V. Hayden was carrying on a 

 series of most energetic explorations in the West, 

 especially in Nebraska and Dakota as then delimited, 

 returning from each trip laden with fossils which were 

 given to Leidy for determination. The results appeared 

 in 1869 in Leidy 's Extinct Mammalian Fauna of Dakota 

 and Nebraska, published as volume 7 of the Journal of 

 the Philadelphia Academy. In this large volume no fewer 

 than seventy genera and numerous species of forms, 

 many of them new to science, were described, repre- 

 senting many of the principal mammalian orders ; horses 

 were, however, especially conspicuous. This last group 

 led Leidy to the conclusion, afterward emphasized by 

 Huxley, that North America was the home of the horse in 

 geologic time, there being here a greater representation 

 of different species than in any recent fauna of the 

 world. Leidy 's interest in the horses, for the forward- 

 ing of which he made a large collection of recent mate- 

 rial, extended over many years, as his first paper on the 

 subject bears the date of 1847, the last that of 1890. 



Next came the discovery of Eocene material from the 

 vicinity of Fort Bridger, Wyoming, geologically older 



