162 AN ATTEMPT TO DESCRIBE THE ANIMALS THAT MADE 



12. Width of the toes. — I have attempted to ap])ly this charac- 

 ter only to the pachydactylous tracks, as the others are so nearly 

 alike, and so narrow, that no importance would attach to the meas- 

 urements. The numbers given in the description of the several 

 species of thick-toed animals are obtained from the same speci- 

 men, and merely indicate the greatest and least breadth of the 

 phalangeal protuberances. Usually these measurements can be 

 made with a good degree of accuracy, and therefore this character 

 is one of considerable importance. 



The following numbers express the ratio between the average 

 width of the toes in these several tracks, and the length of the 

 foot : — 



Brontozoum giganteum ..... 8.2 



" Sillimanium ..... 10.0 



" expansum ..... 5.8 



" gracillimum ..... 6.2 



" parallelum ..... 7.5 



./Ethyopus Lyellianus . . . . .5.8 



" minor . ...... 5.3 



It is clear that the great differences in these ratios cannot be 

 explained by inaccuracies of measurement ; and hence the thick- 

 ne'ss or breadth of the toes is a good character by which to dis- 

 tinguish species ; as, indeed, an inspection of the outlines of the 

 pachydactylous tracks on Plates 1 , 2, and 3, will evince. 



13. Number and length of the phalangeal expansions. — These 

 points can of course be determined only in the thick-toed spe- 

 cies ; but then they are of great importance, especially the number 

 of expansions on different toes ; for in living animals it is well 

 known that this character determines sometimes the class to which 

 an individual belongs, and in the fossil footmarks this is the main 



