22 ICHNOGRAPHS OF THE 



The ornithic footprints are, as has been remarked, essentially tridactylous ; a 

 small proportion of the waders only, exhibit the fourth toe, which is fixed upon 

 the metatarsal bone at a point above the plane of the anterior feet. It is 

 directed backward and inward as in the living tribes, and in some instances the 

 extremity only of its nail touches the earth (PI. 5). Its impress is rarely seen 

 except in those examples of footprints wherein the bird sank deeply in the soft 

 materials of the stratum, bringing it down to the level of the surface. 



An identical system of articulation of the toes is common to all the ornithoid 

 footprints; and this system is also identical with the corresponding organs of 

 living birds. The number and arrangement of the digital phalanges are uniformly 

 the same in each type of birds, and there is a like agreement in the tarsal 

 and ungual systems; and this uniform correspondence constitutes the basis upon 

 which the affinities and relations of the extinct animals can be comprehended. 

 In both extinct and living races the imier toe is invariably shortest, the middle 

 longest, while the outer holds an intermediate grade. There is no exception to 

 this rule among the fossil footprints. In existing tridactylous birds there are 

 three phalanges of bones for the inner toe, four for the middle, and five for the 

 outer; but as the terminal phalanx belongs exclusively to the nail, there are in 

 the footprint two concave depressions for the inner toe, three for the middle, 

 and four for the outer, that correspond to the tuberous expansions of the respective 

 articulations (PI. 1, figs. 2 and 3, and PI. 2, figs. 1, 2, and 3, impressions of living 

 birds, and PI. 11 and others, fossil impressions). There are, in addition to the 

 phalangeal and ungual markings in well-defined examples of footprints, a group 

 of three concavities, situated posteriorly to the toes (PI. 4, fig. 1), that are 

 impressed by the tubercles of the metatarsal bones that support the respective 

 toes. There is usually considerable variation in these features of the footprint ; 

 they are wanting altogether in the digitigrade examples, and in Plate 7 they 

 are blended so as to destroy their outlines. The central tubercle, or that corre- 

 sponding to the middle toe, is also modified by compression of the first phalanges 

 of the respective toes, that embrace it (PI. 10), and change its form. It is 

 also unusual for the tubercle supporting the inner toe to be impressed at all 

 (PI. 4, fig. 4), and in some forms of the struthoid examples the impress of the 

 tubercle supporting the outer toe is very massive and preponderating (Pis. 9, 

 11, and 13). The nails terminating the toes are broad at their contact with 

 the last phalanx, and are stout and blunt; and, in some very rare instances, the 

 dermoid papillous elevations are distinctly impressed (PL 16). 



