FOSSIL FOOTPRINTS OF THE JURA-TRIAS. 473 



tips of the digits, curiously paralleling Chirotherium barthl of Saxony (pi. 72, figs. B, 

 G). The fad: that both tracks are found in such soft shale that the impression can 

 rarely be preserved lends color to the belief that they were at least seraiaquatic in their 

 habits. Another explanation of the web is suggested, namely to give the creature addi- 

 tional support in soft mad which would have its analogy in the snowshoe. 



(c) Bipedalism. E\ddeuce of bipedalism among dinosaurs is showu in many 

 instances. In Anchisauripus the track during swift movement is tridactyl, the hallux 

 impressing only when the creature was stopping or when the mud was especially soft. 

 The tail "in this genus never impresses but was e\idently used as a counterpoise, or 



balancing organ. 



Anomoepus, though tridactyl and truly bipedal in gait, nevertheless occasionally 

 rested the hands upon the ground and might also drag the tail and impress the hallux for 

 a short distance before stopping. 



Otozoum was a huge, bipedal form, plantigrade and tetradactyl in gait, rarely resting 

 the hands and sometimes dragging the tail as did Anomoepus. 



Batrachopus was quadrupedal with a tetradactyl, very dinosauroid pes, and a much 

 smaller, functionally tetradactyl inanus. The very long, stilted limbs in this genus 

 suggest the Dinosauria as well. 



Finally in Chirotherium one finds a truly quadrupedal form, pentadactyl in both the 

 manus and pes though often the former impresses so lightly as to make a tetradactyl 

 track, as the greater part of the creature's weight was borne on the larger hind feet. 

 This illustrates the statement of Osborn (:00, p. 796) in which he says that " tridactylism 

 is correlated, with rapid bipedal progression, the inner and outer digits suffering reduc- 

 tion." Among modern forms, Chlamydosaurus, an Australian heard (Kent, '97) gives 

 us the l)est illustration of Osborn' s law, though many other lizards, when frightened, run 

 on the liind limits with the tail held as a counterpoise. 



{d) Possible avian footprints. The difficulty of distinguishing a track thus formed 

 from that of a bird led Baur ('87, p. 102) to suppose that Hitchcock was correct in believ- 

 ing that many of the sandstone tracks were made by true birds. It seems to the writer 

 that, with the possible exception of some of the leptodactylous or caved-in tracks whose 

 affinities are very obscure, one can by a method of elimination narrow the possible avian 

 tracks down to the genus Grallatin-. In this group the relative length of 'foot to limb 

 seems to be about that of the bustards, but the foot pads are much more specialized in 

 the bird, those of the outer digit having coalesced to form one as iu the struthious birds. 

 Otherwise the foot of Grallator and that cf Otis are very similar, even to the loss of the 

 hallux. The fact that Grallator is cursorial rather than arboreal does not lessen the possi- 

 bility of its being a bird, for the use to which the ostrich puts its wings while running 



