231 



The tibia pertains to a carnivore, and resembles that of an otter more than 

 that of any other animal with which I have an opportunity of comparing it. 

 Its differences, excepting size, are trifling. The tubercle for insertion of the 

 quadriceps extensor is less prominent, so as to give the head of the bone pro- 

 portionately less thickness in relation with its breadth. The ridge for the 

 attachment of the interosseous membrane at the lower part of the bone is 

 more prominent and sharper. The distal end in front just above the articu- 

 lation is flatter, and the groove for the flexor tendons behind is deeper. 



Liues. 



Length of the bone internally 59 



Width of the head 15 



Thickness at the inner condyle lOi 



Width of the distal end between the most prominent points 11 



Thickness at the inner malleole 8 



Order Proboscidea. 



MASTODON. 

 Mastodon obscurus. 



Besides the well-known American mastodon, M. americanus, of the post- 

 Tertiary period, there appear to have been at least three others which in- 

 habited this continent. Characteristic remains of a species, to which the 

 name of M. mirificus was given, were discovered by Professor Hayden, in 

 association with an abundance of remains of many other extinct animals in 

 the Pliocene formation of the Loup iFork of Platte River, Nebraska. Re- 

 mains also, apparently of the same species as the South American M. andium, 

 have been found in Central America. For an account of the remains of the 

 two species last named, the reader is referred to the "Extinct Mammalian 

 Fauna of Dakota and Nebraska." 



In the Museum of the Academy of Natural Sciences of Philadelphia, 

 there is a cast in plaster of a mastodon tooth, the original of which is reputed 

 to have been found in the Miocene formation of Maryland. The original 

 specimen having been lost, the cast is represented in Fig. 13, Plate XXVII, 

 of the work just named. This, together with the fragment of a similar 

 tooth, represented in Fig. 16, of the same plate, has been taken in evidence 

 of the existence of a fourth species, to which the name of M. obscurus has 

 been given. 



Dr. Lorenzo Gr. Yates, of Centreville, Alameda County, California, has 

 communicated to the writer a list of localities in which he has discovered re- 



