4:54 



FOSSIL FOOTPKINTS. 



[Oh. XXn. 



we follow a single line of marks we find them uniform in size, and 

 nearly uniform in distance from each other, the toes of two successive 

 footprints turning alternately right and left (see fig, 491). Such 



Fig. 491. 



Footprints of a bird. Turner's Falls, Yalley of the Connecticut. (See Dr. Deane, 

 Mem. of Amer. Acad., vol. iv. 1849.) 



single lines indicate a biped ; and there is generally such a deviation 

 from a straight line in any three successive prints, as we remark in the 

 tracks left by birds. There is also a striking relation between the 

 distance separating two footprints in one series, and the size of the 

 impressions ; in other words, an obvious proportion between the 

 length of the stride and the dimension of the creature which walked 

 over the mud. If the marks are small, they may be half an inch 

 asunder ; if gigantic, as, for example, where the toes are 20 

 inches long, they are occasionally 4 feet and a half apart. The 

 bipedal impressions are for the most part trifid, and show the same 

 number of joints as exist in the feet of living tridactylous birds. 

 Now, such birds have three phalangeal bones for the inner toe, four 

 for the middle, and five for the outer one (see fig. 491) ; but the im- 

 pression of the terminal joint is that of the nail only. The fossil 

 footprints exhibit regularly, where the joints are seen, the same num- 

 ber ; and we see in each continuous line of tracks the three-jointed 

 and five-jointed toes placed alternately outwards, first on the one side 

 and then on the other. In some specimens, besides impressions of 

 the three toes in front, the rudiment is seen of the fourth toe behind. 

 It is not often that the matrix has been fine enough to retain impres- 

 sions of the integument or' skin of the foot ; but in one fine specimen 

 found at Turner's Falls on the Connecticut, by Dr. Deane, these 

 markings are well preserved, and have been recognized by Professor 

 Owen as resembling the skin of the ostrich, and not that of reptiles.* 

 Much care is required to ascertain the precise layer of a laminated 

 rock on which an animal has walked, because the impression usually 

 extends downwards through several laminae ; and if the upper layer 

 originally trodden upon is wanting, the mark of one or more joints, 

 or even in some cases an entire toe, which sank less deep into the soft 

 ground, may disappear, and yet the remainder of the footprint be 

 well defined. 



* This specimen was in the late Dr. Mantell's museum, and indicated a bird of a 

 size intermediate between the small and the largest of the Connecticut species. 



