THE FOSSIL FOOTMARKS OF THE UNITED STATES. 153 



argument that leads to the conclusion that some of them were 

 made bj birds. 



In estimating the number of phalanges from the tubercular ex- 

 pansions in the footmarks, I have supposed that the ungual and 

 penultimate phalanges would make but one impression ; and in 

 general this conclusion is borne out by an examination of the feet 

 of living animals. 



It is also sometimes difficult to distinguish between impressions 

 made by the phalanges, and those of the metacarpal or metatarsal 

 bones. The tracks of the anomalous Otozoum Moodii exhibit this 

 difficulty more distinctly than any other, as the detailed description 

 of that species will show. Plate 12. 



The number of phalangeal impressions on the tracks is greatest 

 in the outer toe in all cases yet met with ; and hence they are 

 usually less distinct there, — so indistinct often that their measure- 

 ment is difficult ; and, indeed, the mere length of these impres- 

 sions has not as yet been applied as a generic or specific dis- 

 tinction. 



14. Character of the heel. — The fossil footmarks show much 

 variety in this part, and being a constant part, it is of much 

 value in determining the nature of the animal. In very many 

 cases, the metacarpal or metatarsal bones seem to have been placed 

 in so oblique a position, that neither they, nor the integuments be- 

 neath them, reached the ground ; and we have accordingly only the 

 imprint of the toes, as in Platypterna tenuis (Plate 7, fig. 2) and 

 Argozoum minimum (Plate 6, fig. 5) ; that is, the feet were digiti- 

 grade. Indeed, in some cases the middle toe seems to have been 

 articulated so high to the metatarsus or metacarpus, that it reached 

 the ground only a good deal in advance, a striking example of 

 which is shown in the Typopus abnormis (Plate 10, fig. 6). 



21 



