686 The Smithsonian Institution 



apparently nearly related to those of the Jurassic beds of 

 North America, while three of the species bring to mind 

 most strongly the species which now inhabit Europe and 

 western Asia, and a small group belonging to the Mississippi 

 area. The variety of characters displayed by these Triassic 

 Unios go to show that the genus must have been well estab- 

 lished at the time the Dockum beds were laid down, thus 

 tending to overthrow Neumayer's theory that the Unionidae 

 were derived from the genus Trigonia, which probably does 

 not date back to a period earlier than that of the shells under 

 consideration. 



The first work published by the Institution in vertebrate 

 paleontology was a memoir of the Mosasaurus and three 

 new genera, Holcodus, Conosaurus, and Amphorosteus, by 

 Robert W. Gibbes, and was issued in 1850. 



The material upon which this memoir is based was found 

 in the Cretaceous and Eocene marls of South Carolina and 

 Alabama. The review of the literature of the subject is 

 accompanied by two plates, showing the five species of Mos- 

 asaurus then known, and a third displaying the three new 

 genera. This memoir was a quarto. 



In 1852 Joseph Leidy's "Memoir on the Extinct Species 

 of American Ox" was published, and was followed in 1853 

 by his celebrated report upon the Bad Land collections, enti- 

 titled "Ancient Fauna of Nebraska." Both of these works 

 belong to the " Contributions to Knowledge " series, and are 

 handsomely illustrated. 



In the first paper Doctor Leidy indicates the former existence 

 of four species of ox, which were probably contemporaneous 

 with the Mastodon and the Megalonyx. Fossil remains of 

 these animals had been frequently found in the United States, 

 and descriptions of them are scattered through various works; 

 but no approach had before been made to a correct view 



