ACADEMY OF SCIENCES.] PUBLISHED RESULTS. 123 



minor papers on additional anatomical features and a thoughtful article on the relationships and 

 habits appear subsequently. The 1S9S memoir is a work of monographic value and complete- 

 ness, as a resume of its contents will show. The opening pages are an historical summary of the 

 work previously done on the mosasaurs. Succeeding this comes a section on the range and dis- 

 tribution of the group and a generic and systematic summary. This is followed by an elaborate 

 series of comparative anatomical descriptions, in which the homologies of the cranial and other 

 skeletal elements are discussed with that detail and degree of accuracy of which Williston's 

 broad anatomical knowledge made him master. 



Next come systematic descriptions and a discussion of the biology of the group, with restora- 

 tions. He speaks of the creatures as marine lizards of moderate size, ranging from 10 to perhaps 

 37 or 38 feet, living in shallow waters, although some of the larger of them ventured far out to 

 sea. Their feeding habits, as evidenced from the very peculiar lower jaw which had a joint in its 

 mid-length, are discussed, but Williston felt that the rigidity of the breast girdle precluded any 

 very remarkable feats in the way of swallowing bulky prey. He believed that the food consisted 

 of the numerous small fishes which swarmed the seas with them — possibly an occasional young 

 mosasaur such as, curiously enough, is almost unknown as a fossil. He believes, further, that the 

 mosasaurs rarely came ashore, although they must have done so for egg laying, as there is no 

 evidence that they were viviparous, but the body is not sufficiently serpentine, nor the limbs 

 sufficiently strong, for terrestrial locomotion. They were very pugnacious, as numerous ex- 

 ostoses on the skeleton show. The body was covered with a scaly skin, the scales closely resem- 

 bling those of a large monitor in size and shape. This knowledge is based upon a specimen of 

 Tylosaurus in which the carbonized scales are present on the anterior part of the body. Not 

 only are the skeletal restorations of the three principal genera given, but a restoration in the 

 flesh of Clidastes velox, with the associated TJintacrinus and Omithostoma (Pteranodan) ingens. 



The publication of this memoir, supplemented by the researches of Dollo of Belgium, gives 

 us a body of information concerning the mosasaurs as authoritative as it is complete, and one 

 which has served as a basis for all subsequent research upon the group. 



PLESIOSAURS. 



The plesiosaurs of the Niobrara also naturally attracted Williston's attention, and he 

 published his first paper on the group in 1890, 5 describing a new species, Cimoliasaurus (Elasmo- 

 8aurus) snowii, in which he pays particular heed to the hitherto little known skull. Another 

 paper, based upon the same specimen, appeared in Science about the same time. 6 In 1893, 13 

 Williston wrote on an interesting food habit of the plesiosaurs, but did not discuss them again 

 until 1897, 28 when he described another new form from the Kansas Cretaceous (Comanchian) . 

 In 1902 57 there appeared a morphological paper on the plesiosaurian cranial elements and 

 still another 59 on the restoration of Dolichorhyn chops osborni, a new Cretaceous plesiosaur. 

 This latter skeleton was worked out of the matrix and mounted, and is to-day one of the very 

 few such free mounts in the country. 



The paper on certain homoplastic characters in aquatic air-breathing vertebrates, 61 pub- 

 lished simultaneously with the one just mentioned, is the forerunner of Williston's much later 

 work on "Water Reptiles of the Past and Present." In the former he sums up the general 

 lines of adaptation which all secondarily adapted aquatic vertebrates must follow, and includes 

 some interesting observations on the plesiosaurs, their rather clumsy form, lack of speed, and 

 feeding habits, for he believed that many of them fed largely upon cephalopods and other 

 invertebrates, as did the ichthyosaurs; the plesiosaurs, however, lived in relatively shallow water, 

 as compared with the ichthyosaurs. He speaks here again of the stomach stones, later called 

 gastroliths, found in abundance with the remains of plesiosaurs, and, in some instances, carried 

 several hundred miles from their source. He held, however, that the plesiosaurs differed not 

 a little among themselves in habits, as evidenced by their great variations in form. They 

 exceeded in size the largest mosasaurs, but were never a match for the latter in prowess or 

 voracity. Like the ga vials, they were comparatively harmless creatures. "Were they living 

 59490°— 24 9 



