0. C. Marsh — Notice of a 'New Fossil Sirenian. 95 



Desmostylus hesperus, gen. et sp. nov. 



The remains known of the present species indicate an animal 

 about fifteen feet (M. 4*5) in length, and of robust proportions. 

 The most characteristic parts preserved are the molar teeth, 

 which are composed of a number of vertical columns, closely 

 pressed together, and in adult animals, firmly united at their 

 bases. These columns are thickly invested with enamel, 

 which is rugose externally. Inside the enamel, is a body of 

 dentine, in which there is a central cavity. 



In immature teeth, the columns are nearly round, and loosely 

 united, but as they increase in size, they press together, and be ■ 

 come more or less polygonal in cross section. Before being 

 worn, they have their summits smooth and convex, but after 

 some use, the center of each column presents a rounded eleva- 

 tion, well shown in the figures below. This is due to the 

 harder material forming the walls of the central cavity. As 

 this apex is removed by further wear, the cavity is reached, and 

 this central opening increases in size as the tooth is shortened 

 by attrition. 



Fig. 2. 



Fig. 3. 



Figure 1. Part of tooth of Desmostylus hesperus, Marsh: 

 Figure 2. The same specimen; seen from above. 

 Figure 3. The same specimen ; inner surface. 

 All the figures are natural size. 



end view. 



The specimen figured is apparently the posterior portion of 

 a molar tooth. The three columns shown are much smaller 

 than the average, not half as large as some others found with 

 them, and probably belonging to the same individual. The 

 number of columns in a single tooth is uncertain, but there are 



