THE FOSSIL FOOTMARKS OF THE UNITED STATES. 157 



of the latter, the genera Argozoum and Platypterna, on Plates 6 

 and 7, furnish examples. 



But there is an intermediate and remarkable variety, in which 

 the heel and toes made a deep impression, but a space between 

 them is left unimpressed, and not unfrequently rising above the 

 original surface, either in a curve or a ridge. We have examples 

 of this in Steropezoum ingens and elegans (Plate 5), in Harpedac- 

 tylus concameratus (Plate 14, fig. 3), and in Tricenopus Baileyanus 

 and Emmonsianus (Plate 10, figs. 4 and 5). In such cases it can- 

 not be doubted that the long os calcis, or sometimes perhaps the 

 carpal or tarsal bone, which formed the heel, was so articulated to 

 the other bones of the foot as to constitute an arch, or even to 

 form an angle, considerably acute, as in some quadrupeds ; so that 

 when the mud was impressed by the heel and the toes, it would 

 be crowded upwards between them. This would exactly explain 

 the appearance of some of the tracks above referred to ; and it 

 gives us an accurate view of the character of the bottom of the 

 foot, and to some extent of its osseous structure. Sometimes the 

 elevation of the rock, behind the toes, is irregular ; indicating a 

 corresponding irregularity on the bottom of the foot, as in Sterope- 

 zoum elegans, Plate 5, fig. 2. 



16. Versed sine of the curvature of the toes. — Some species of 

 the footmarks are remarkable for the curvature of the toes. In the 

 tracks with three toes directed forward, the middle toe always 

 curves towards the line of direction on which the animal was ad- 

 vancing, and the lateral toes usually curve outwards near their 

 tips. (See the figures of Steropezoum ingens and elegans, Argozoum 

 Redfieldianum, the species of Platypterna, and especially of Orni- 

 thopus loripes, Plate 5, figs. 1 and 2, Plate 6, fig. 1, Plate 7, 

 figs. 1-4, and Plate 8, fig. 3.) 



