THE FOSSIL FOOTMARKS OF THE UNITED STATES. 149 



6. Angle made by the inner and middle toe, and the outer and 

 middle toe. — These angles are perhaps not quite as constant as that 

 between the lateral toes ; for in treading upon the mud, the strain 

 upon the foot seems sometimes to have varied a little the position 

 of the middle toe. Still, this character ought not to be neglected. 

 In some instances, the curvature of the toes is so great, that it is 

 difficult to measure the angles described under this and the preced- 

 ing heads. But I have made it a rule to draw the lines forming 

 the angles, from the middle of the toes, at their origin, to their tips. 



7. Projection of the middle toe beyond the lateral ones. — This is 

 not exactly equivalent to the difference in length between the mid- 

 dle and lateral toes, because the middle toe generally does not 

 reach backward so far as the others. It is an important and con- 

 stant character, and serves to distinguish several species ; as the 

 Argozoum dispari-digitatum from the A. pari-digitatum. 



8. Distance between the tips of the lateral toes. — This is deter- 

 mined by the angle of divarication and the length of the lateral 

 toes ; but as it would need the solution of a case in trigonometry, 

 it is easier to measure the distance ; for it is useful in comparing one 

 track with another. 



9. Distance between the tips of the middle and the inner and 

 outer toes. — These elements are also determined by the previous 

 ones ; but it is more convenient to measure than to calculate them. 

 It is obvious that they are among the permanent characters, and 

 therefore useful for settling the genus and species. 



10. Position and direction of the hind toe. — This character ap- 

 plies only to those tracks that have three toes directed forward, and 

 a single one behind. And it is obvious that the latter may have 

 a great variety of positions and directions, and furnish, therefore, 

 (since these characters are constant in the same species,) good 



