THE FOSSIL FOOTMARKS OF THE UNITED STATES. 151 



length it lies nearly on a line of the outer toe backward, which is 

 a characteristic of another genus, the Ornithopus ; and as the heel 

 frequently disappears, the track is likely to be confounded with the 

 Ornithopus gallinaceus (Plate 8, fig. 1), although generally they 

 appear very much unlike. This singular change of position in the 

 hind toe I find it very difficult to explain by any of the hypotheses 

 which I have suggested above, in describing the fourth character. 



11. Character of the daw. — This embraces its length and 

 width ; yet, with one exception, the length only is noticed. In the 

 genus jEthyopus, the width of the claw indicates, if I mistake not, 

 that it was winged. It is only in the pachydactylous tracks that 

 the length of the claw, if it existed, can be ascertained, except 

 in the Argozoum Redfieldianum, where a single specimen reveals 

 it ; and I doubt not it exists in all the leptodactylous feet, whose 

 extremities are always acuminated. 



The ratio between the length of the claw and that of the foot, 

 in all the species where claws have been measured, is as follows: 



Brontozoum giganteum ..... 9.9 



" Sillimanium • • . . . 6.75 



" expansum • ■ . . . 5.9 



" gracillimum ■ • . . . 6.2 



" parallelum . . . . . 8.1 * 



jEthyopus Lyellianus ..... 6.2 



" minor • . . . . 5.7 



Argozoum Redfieldianum ..... 6.2 



These numbers do not differ from one another more, perhaps, than 

 can be explained by uncertainties of measurement, which in the 

 case of the claw must be considerable. Hence we may conclude 

 that the length of the claw varies in the same proportion as that 

 of the foot ; at least, as nearly so as in living animals. 



