MAMMALIA OF THE SANTA CRUZ BEDS. 

 PART I. LITOPTERNA. 



BY 



WILLIAM B. SCOTT, 

 Princeton University. 



THE memoir on the Litopterna was originally to have been written 

 by the late Mr. Hatcher, but through his lamented death, the work 

 has devolved upon myself. Unfortunately, during my visit 

 to La Plata, my limited time was so fully taken up with other groups of 

 Santa Cruz fossils, that it was impossible for me to make any satisfactory 

 studies of the Litopterna in the collections of Dr. Ameghino and the La 

 Plata Museum. This lack has, to some extent been repaired by the kind- 

 ness of Dr. Ameghino, who has with his usual courtesy sent me excel- 

 lent photographs and plaster-casts of nearly all of his types of the Santa 

 Cruz Proterotheriidae and these have proved very helpful in the deter- 

 mination of the genera and species. 



The materials for the study of this group in the collections of Princeton 

 University and the American Museum of Natural History are abundant 

 and admirably preserved, and a very full account of the dental and skeletal 

 structure may be given. On the other hand, the determination of species, 

 more particularly of the genus Proterotherium, is exceeding difficult. To 

 solve the problem of species in a satisfactory and convincing manner, it 

 would first be necessary to have a detailed stratigraphical knowledge of 

 the Santa Cruz formation and, further, to ascertain the exact position in 

 the beds of every specimen collected. In the absence of such knowledge, 

 it is not yet practicable to distinguish contemporary and fluctuating varia- 

 tions from successive and relatively fixed mutations, and where a genus was 

 in a state of vigorous development, the number of species which should 

 be assigned to it becomes largely a matter of conjecture. For the most 

 part, the genera of the Santa Cruz Litopterna are well-defined and easily 

 distinguishable, save such as are still very incompletely known, and the 

 families are even better and more clearly distinguished. 



