154 AN ATTEMPT TO DESCRIBE THE ANIMALS THAT MADE 



A more common case is where the cushion beneath the meta- 

 carpal or metatarsal bones made an impression, but the bones 

 themselves left no distinct imprint. This was usually the case 

 with the pachydactylous tracks. But in two species at least, viz. 

 the Brontozoum Sillimanium and B. parallelum, a distinct impression 

 remains of the double-headed extremity of what was probably a 

 tarso-metatarsal bone (Plate 3, figs. 2 and 4) ; for, besides these two 

 rounded impressions, we have the four others in the outer toe 

 which all the other tracks exhibit. Many of the leptodactylous 

 tracks exhibit an impression of the cushion beneath the bones that 

 lie behind the toes, forming a heel which slopes upward and back- 

 ward so gradually, that it is impossible to say exactly where it 

 terminates. For the mud yielded a little beyond the margin of 

 the track, and this fact, in many instances, is a great hindrance to 

 finding out the exact size and shape of the foot, and moreover 

 is the grand difficulty of giving a satisfactory representation of 

 these tracks. For this reason, I have in many instances, in 

 the accompanying sketches, left the posterior part of the heel 

 without an outline ; as in Platypterna tenuis, Ornithopus Adam- 

 sanus, and some others. 



In other cases, the posterior margin of a rounded heel is strongly 

 marked, not, as we might at first suppose, because the animal sunk 

 deeper on account of the peculiar state of the mud, but because it 

 was a heavier animal, and one that trod more upon his heel ; for 

 we find the same deep impression wherever it trod. Examples of 

 this sort are Polemarclms gigas, Palamopus Dananus, and some- 

 times Trianopus Emmonsianus, Plates 9, 10, and II. 



A few species present us with a heel of a very peculiar character, 

 of whose exact nature I am yet in doubt. Just behind the point 

 where the toes originate, the surface in the track rises above the 



