124 SAMUEL WENDELL WILLISTON- LULL. [MEM °7vou™-n; 



to-day, [they] would find unopposable foes in the vicious and cruel crocodiles. They were 

 relatively stupid and slow, cruel enough to the smaller creatures, but of limited prowess. But 

 in structure and habits they are among the most remarkable of all the animals of the past 

 or present." (Water Reptiles, p. 76.) 



Williston's most extensive memoir on the plesiosaurs appeared in 1903," as a publication 

 of the Field Columbian Museum, during the time he served that institution as associate curator 

 of the division of paleontology, in addition to his professorial duties in the University of Chicago. 

 This paper was called Part I, and was to have been followed by another, which, however, never 

 appeared. He speaks of the great number which have been described from the United States, 

 32 species and 15 genera, of which in not a single instance has there been even a considerable 

 part of the skeleton made known, while the skull is known in but three, and only one of these 

 has been described. And yet plesiosaurs are not at all rare in American deposits or collections. 

 The monographic studies on the group, undertaken by Williston, were for the purpose of clearing 

 up the confusion which then existed concerning these animals, but he deemed it wise, as there 

 was still much to be done, to publish his detailed researches on the three species Dolichorhynchops 

 osbomi, Brachcmchenius lucasii and Cimoliasaurus snowii, rather than wait for the completion 

 of the entire work, and it was fortunate that he did so, in view of the fact that the second part 

 did not appear. This was to contain the descriptions and illustrations of two or three other 

 skulls. 



The origin of the plesiosaurs Williston also proposes to discuss in a later paper. Here he 

 contents himself with merely saying that he believes their nearest affinities among all reptiles, 

 recent or extinct, to be with the dicynodonts (see table facing p. 133). After the introduction 

 there is a catalogue and brief bibliography of the North American plesiosaurs, followed by the 

 morphological description of the three species mentioned. Another new species is described, 

 Polycotylus iscliiadicus, a further description of Plesiosaurus gouldii Williston is given, propodial 

 bones of young plesiosaurs which are abundant in the Chalk are described, and the essay closes 

 with a discussion of the peculiar food habit of the plesiosaurs of which he also wrote in 1902. 

 This last brought forth a discussion from Dr. C. R. Eastman which was met by Williston in 

 a rejoinder in Science for October 22, 1904. 69 



The plesiosaurs are still further discussed in a paper entitled " North American Plesiosaurs : 

 Elasmosaurus, Polycotylus and Cimoliasaurus," appearing in 1906. 78 In this instance, the study 

 was based largely upon material in the Yale Museum, some of which had been collected and studied 

 by the author himself twenty years or so before. Prof. Marsh had begun a critical study of the 

 Yale material, hence much of it was prepared and some illustrations made, all of which, through 

 the courtesy of the curator, Prof. Schuchert, were placed at Williston's disposal. The genus 

 Cimoliasaurus is denned as distinct from the long-necked Elasmosaurus. Of the latter, a number 

 of species, two of them new, are described. The genus Polycotylus is also clearly diagnosed, and 

 Trinacromerum is believed to be distinct from Polycotylus although the reasons for this belief 

 are not given. No generalizations are included in the paper. 



Two years later, 86 Williston again published on the genus Trinacromerum, mainly a morpholo- 

 gic description of the type species, T. bentonianum, to which he adds another new species, 

 concluding with a summation of the family Polycotylidas, which is defined, with a list of genera 

 and species and their bibliography. Finally there is a list of the described North American 

 plesiosaurs, 36 in all, arranged in stratigraphical sequence. 



A paper on Brachauchenius, published in 1907, 79 discusses not only the characters of this 

 remarkable genus, but the relationships of the plesiosaurs as a whole. In discussing similarities 

 between them and the turtles, Williston concludes that such as they show are due solely to parallel 

 evolution, and that there is only a remote relationship between the two orders in osteological 

 structure. 



"The plesiosaurs," he says (p. 489), "could not have been derived from any ancestors that might by the widest 

 stretch of imagination be called Chelonia, or Chelonia-like. Nor could the turtles have come from any forebears even 

 suggesting the sauropterygian structure. I am still strongly of the opinion that the Sauropterygia were derived from 

 a primitive therocephalian ancestry; while I am firmly of the opinion that the turtles have had a quite independent 



