THE FOSSIL FOOTMARKS OF THE UNITED STATES. 159 



those whose feet diverge from the axis of the body, the divarication 

 between these lines may be quite large. Nay, in some reptiles 

 (ex gr. Algyra barbarica, Griffith's Cuvier, Vol. IX., p. 212, rep- 

 resented on Plate 23, fig. 6, of this paper), the hind foot 

 is so situated, that it makes a very obtuse angle with the line 

 of direction ; and, in fact, the hind and fore feet point in nearly 

 opposite directions ; so that from the tracks alone one cannot de- 

 termine in which direction the animal moved. It is obvious, then, 

 that this is an important character, sufficient to distinguish species, 

 and even genera. 



18. Distance of the middle of the heel, or posterior part of the 

 foot, from the line of direction. — I might have selected the tip of 



the longest toe as the point from which to measure, instead of the 

 middle of the heel. But whichever extremity of the foot is used, 

 the position of the other end is fixed, if we know the divarication 

 between the axis of the foot and line of direction. And it is ob- 

 vious that the distance to the right and left of the line of direction, 

 at which we find the tracks, will depend partly and mainly upon 

 the distance between the points of insertion of the legs upon the 

 animal's body, and partly upon their length. Hence it must be a 

 constant character, and cannot vary much in the same animal, ex- 

 cept, perhaps, in some of the sprawling quadrupeds. I have never 

 depended upon it alone to distinguish species ; but I think it might 

 be safely done, when the character is well marked. 



19. Length of the step. — By running the eye over the column 

 which shows the ratio between the length of the foot and the step, 

 in the table of the characters of species, annexed to this paper, 

 it will be seen that there is a general correspondence between the 

 length of the foot and of the step. Yet the differences in the 

 ratios make it equally obvious, that some of the animals were short- 



