684 HENRY FAIRFIELD OSBORN 
inferior material, marks the turning-point in this field to the new 
order of description and generalization based upon complete 
material, including even the skin impressions of several great 
mosasaurs. In his observations on the mosasaurs, plesiosaurs, 
_ pterodactyls and marine turtles, and the birds with teeth, Odon- 
tornithes, he placed the osteology of these several animals on a much 
more secure basis, adding a number of new generic types, such as 
a short-necked plesiosaur, Dolichorhynchops osbornt. 
His interpretation of function and habit is shown in his restora- 
tions of all these types, and his first observations on the feeding 
habits of the plesiosaurs and his more mature views on several of 
these animals were published during his sojourn in the University 
of Chicago, namely, “Relationships and Habits of the Mosasaurs,”’. 
Journal of Geology, 1904; “North American Plesiosaurs,’’ 1903, 1906, 
1907. His first contribution to the phylogeny and classification of 
the Reptilia as a whole appeared in 1905 and was followed by his 
important discussion of this subject entitled ‘‘The Phylogeny and 
Classification of Reptiles,” Journal of Geology, August, 1917. In 
this article, which expresses his mature opinions, he departed from 
his previous conservative attitude toward classification and pro- 
posed to add two subclasses of reptiles, the Anapsida and Parapsida, 
to the subclasses previously proposed by Osborn, namely the Synap- 
sida and the Diapsida, making a fourfold grand division of the 
Reptilia. Doubtless it was Williston’s intention to fortify this 
system of classification in his forthcoming general work on the 
Reptilia. 
WORK ON PRIMITIVE AMPHIBIANS AND REPTILES* 
In 1902, at the age of fifty, Williston was called to the University 
of Chicago as head of the new department of vertebrate paleon- 
tology, a chair which he occupied with great distinction and with 
continued influence for the remaining sixteen years of his life. He 
now began to concentrate his attention more exclusively on verte- 
brate paleontology. During the first six years he continued his 
studies and publications on the Cretaceous reptiles; then he began 
to turn toward the study of far more difficult and obscure problems, 
namely the relatively primitive amphibians and reptilian life of the 
t See footnote, p. 683. 
