FOSSIL FOOTPRINTS OF THE JUKA-THIAS. 475 



The so called " leptodactylous " tracks of Hitchcock were, as a rule, made in such soft 

 mud that the subsequent caving of the sides of the impression has rendered almost unre- 

 cognizable certain well known species and as a consequence has given rise to new ones 

 which have added to the diiticulty of unraveling the synonymy. Doubtless there were 

 creatures which fre»|uented these soft mud flats whose remains are not found elsewhere, 

 but it is well nigh impossible to fix their true characters. Both "pachydactylous" and 

 "leptodactylous" tracks sometimes occur upon the same slal) which may be accounted 

 for by the supposition that time siitticient for the mud to dry out elapsed l)etween the 

 passage of the two animals. The presence of puddles or tide pools would account for a 

 variable degree of dampness within a comparatively small area. 



The general I'ai'ity of the narrow toed tracks and their abundance in one locality, as 

 at Weatherstield, Connecticut, where the rock is a very flue red shale and nmst have been 

 exceedingly plastic when the prints were formed, makes it extremely hazardous to erect 

 many genera and species upon such remains. 



(c) VaridtioKs of aye hnprolxdjlc. It has been argued that one ought to find the 

 tracks of young among the number and that possibly some of the so called species repre- 

 sent the youthful individuals of forms the adults of which made larger impressions. 

 While this may in certain instances be true, it seems hardly probable for several reasons, 

 among which maj' be mentioned the comparative constancy of size among the manv 

 individuals referable to a single group and the rarity of gradational specimens. 



In some cases the adult condition of the individual is indicated by the fact that the 

 claw impression is offset some little distance beyond the distal pad, the .significance of 

 which can be appreciated by the study of the foot of the cassowary, for example, in which, 

 in the young, the claw and flesh impressions are united, whereas in the adult they are 

 widely separated due to the growth of the claw. It has been suggested that the growth 

 of dinosaurs was like that of trees, continuous throughout life until the senile condition 

 was reached. This is true only of acjuatic, poikilothermous forms in which, as some one 

 has said, the energy which, in the bird or land mammal, is used in overcoming gra\'ity 

 and in the production of heat, can be turned into the growth force. The dinosaurs 

 which frequented these shores were either land forms or, in a few cases, as that of 

 Otozoum, possibly semia([uatic and it is further possible that they were warm blooded. 

 Thus it appears highly probable that they were of limited growth. 



The Sauropoda which probably had this tree-like growth are entirelv ab.sent from the 

 footprint fauna. Hitchcock ('58. p. 41) thinks it hardly probable that " verv youno- 

 animals would often frequent such thoroughfares as the localities of footmai'ks seem to 

 have been .... where so many animals resorted, and where in the dearth of food that 

 nuist sometimes have existed, the young ones must often have been devoured." 



