DAKOTA AND NEBRASKA. 



3il 



"Width of palate between first molars, 



Length of hiatus from first molar to incisives, 



Width of narrowest portion of cranium, 



Depth of lower jaw at first molar, . 



Height of condyle from base at angle, 



Length of lower jaw from condyle to front of incisive alveoli, 



Breadth of jaw from prominence of posterior border to fore part of molar 



alveoli, ..... 

 Diameter of incisors. 

 Diameter first upper molar, 

 Antero-posterior diameter of last upper molar, 

 Antero-posterior diameter of first lower molar. 

 Transverse " " " 



Antero-posterior diameter of last lower molar. 



Lines. 



2i 

 Hi 



4J 



6 

 Hi 

 2H 



14i 

 II 

 II 

 li 

 2 



IJ 

 li 



CASTOR, 



Castok tortus. 



A specimen indicating the former existence of a species of Beaver, about half the 

 size of the recent one, was discovered by Dr. Hayden in the loose sands of the Nio- 

 brara River. Dr. Hayden considers the fossil as belonging to the pliocene period, and 

 attributes it to bed F of his vertical section of the tertiary formations of Nebraska and 

 Dakota. The specimen represented in figure 21, plate XXVI, consists of the greater 

 portion of an upper jaw, containing on both sides the anterior three molars and por- 

 tions of the incisors. It belonged to a quite aged animal, as indicated by the condi- 

 tion of the molar teeth, which are nearly worn away to the fangs. 



The portion of the jaw consists of the under parts of both maxillary and intermax- 

 illary bones, which together are the diminished counterpart in form of the correspond- 

 ing parts of the recent Beaver. 



The incisor teeth likewise have the same relative proportions and form as in the 

 latter animal. 



Of the molar teeth, represented in figures 21, 22, the first is less worn than the 

 others, in consequence of a part of its functions having been performed by a predeces- 

 sor. It bears a near resemblance in its construction and enamel folding to that of the 

 existing Beaver. The succeeding pair of molars appear different from the correspond- 

 ing teeth which I have had the opportunity of examining in skulls of the recent 

 Beaver, but the differences appear to me to be due to difference of age. These teeth 

 have their crowns nearly worn out, and appear to have undergone such a change 

 that what was their inner side has become posterior, as seen in figures 21, 22. I may 



