476 RICHARD SWANN LULL OX 



It is possible that tlie tidal Hats were seasoualh- resorted to as breeding places and 

 that the young were brought forth in remote spots where their footprints would not be 

 preserved, possibly some dry, sandy place at a distance from the water. As a reason for 

 the selection of such rookeries Hitchcock ('58, p. 19) suggests "that the eruption of the 

 trap produced an island ridge in the waters, and the heat of the cooling rock made it a 

 favorite resort for animals, and rapidly dried and therefore preser\ed their tracks in such 

 perfection." 



Taxonomic Characters. 



The following are the characters which are used in the distinction of genera and 

 species in ichnology. 



1. Whether bipedal or quadrupedal in gait. Here one iiuds a gradational series, 

 for the creatures may be absolutely bipedal, never resting the manus upon the ground, or 

 they may be bipedal in gait and rest upon all fours. They may be quadrupedal but mth 

 the bulk of the weight borne upon the hind limbs ; or they may be truly quadrupedal 

 with a more equable di^dsion of the weight over both manus and pes. 



2. The presence or absence and the character of a caudal trace. While the absence 

 of this impression is constant in certain genera, as Auchisauripus and Grallator, its 

 presence is a variable character and does not always appear on every specimen. In 

 Selenichnus and one or two other genera it seems always to be present, but it will readily 

 be seen that it depends largely upon the animal's gait, as in running tlie tail was well 

 raised. 



3. The relative size and contrasted characters of the manus and pes are very impor- 

 tant points in the correlation of the footprint classification with that of the animals as 

 known from other sources. Fore and hind feet are generally clearly recognizable as 

 such, yet Hitchcock ('58, p. 102) in the genus Plesiornis seems to have taken the accom- 

 panying tracks of a somewhat smaller bipedal animal for those of the fore feet of a quad- 

 rupedal form. The pes seems to be generally in a more advanced stage of evolution than 

 the manus and in the dinosaurs to be larger and much more ornithoid. 



4. The distinction of the right and left foot. Where the phalangeal formula is 

 betrayed by the pads, the number of phalanges is always as in lurds, one greater than the 

 number of the digit, thus : I, '/, ^f , ^J. In four toed forms where there is one backwardly 

 pointing toe it is invariably the hallux and hence on the inside of the foot (fig. 7). In 

 the five toed Chirotherium on the contrary the backwardly curved toe, or poicce of the 

 French, represents the fifth digit and is upon the outside of the foot (pi. 72, figs. B, D). 

 In the tridactyl or tetradactyl dinosaur tracks the claws of digits II and IV point awa^- 



