242 Prof. 0. C. Marsh — American Jurassic Mammals. 



second locality of importance is in Colorado, about three hundred 

 miles south of the most northern known limit of these remains. 



The other vertebrate fossils from this horizon are mainly Dinosaurs, 

 many of them of gigantic size, but some scarcely larger than the 

 mammals. Crocodiles, Turtles, small Lizards, and Fishes, are also 

 •well represented. A single Bird (Laopteryx) , and one small Ptero- 

 dactyl, have likewise been recognized from these deposits. More 

 recently, various bones of small, anourous amphibians (EohatracJius 

 agilis) have been found, the first detected in any Mesozoic formation.^ 

 The deposits are lacustrine, as shown by the fresh- water shells they 

 contain. 



In investigating these American Jurassic Mammals, it was neces- 

 sary to compare them, first of all, with those from the same 

 formation in Europe. On this subject, the elaborate memoir of 

 Owen, on British Mesozoic Mammals, was taken as the main 

 authority.^ 



The first specimens discovered in America proved to be very 

 near allies of European forms, and most of those since found show 

 a remarkable resemblance to others described by Owen. Some 

 fragmentary specimens cannot indeed be distinguished from the 

 English fossils, but where the remains are more complete, various 

 differences are seen, which appear to be distinctive. A few well- 

 marked American genera have no known representatives in Europe, 

 while some forms found in Europe are unknown in America. 



One difficulty in the investigation of the remains from the two 

 widely-separated regions arises from the necessity of relying mainly 

 upon figures for comparison. Again, these minute, delicate fossils 

 are often embedded in a matrix from which they cannot be removed 

 without great danger of injury or destruction. Hence, the jaws and 

 teeth in many cases must be examined and described from the single 

 side exposed. If the opposite side of a similar jaw should be shown 

 in another specimen, the two may easily be regarded as distinct. 

 This may also be the case where upper and lower jaws are found 

 separately. Hence, a large amount of material becomes necessary 

 for even a proximate correlation of the closely-related forms. 



Plagiaulacid^. 



One of the first American specimens discovered resembled strongly 

 the minute lower jaws first described by Falconer, under the name 

 Plagiaulax, and since investigated by Owen, Flower, and others, 

 whose discussion of the habits and affinities of those peculiar mam- 

 mals forms a well-known chapter in the history of palgeontology. 

 Of this genus, only the lower jaws were known, and this is one 

 I'eason for the wide divergence of opinion as to the nature of the 

 animals they represent. The lower jaws found in America were 

 regarded by the writer as indicating a distinct genus, Ctenacodon, 

 two species of which he has since described. 



' Proceedings British Association, Aberdeen Meeting, p. 1033, 1885. 

 2 Monograph of the Fossil Mammalia of the Mesozoic Formations, Palseonto - 

 graphical Society, vol. xxiy. London, 1871. 



