144 AN ATTEMPT TO DESCRIBE THE ANIMALS THAT MADE 



an inch in thickness. In this case, the three toes, near their roots, 

 produce the appearance of a heel on the inferior layers; probably 

 because, being so near together, all the mud between them was de- 

 pressed together. Figs. 17 -19 of the same plate exhibit a track 

 of the hind and fore foot of Plectropus longipes, so united as to 

 seem to be only one track. Nor is there any evidence, from this 

 specimen, of two tracks having been made almost in the same spot. 

 But the specimen of the same species, very analogous to this, 

 shown on Plate 10, figs. 1 -3, as seen on different layers, makes it 

 almost certain that they are tracks of the hind and fore foot in 

 both instances. The more detailed account of these specimens 

 will be reserved until I come to describe the Plectropus longipes. 



The above statements show us the great difficulty, in some cases, 

 of ascertaining the precise layer of rock on which the animal 

 walked. Where the surface was considerably firm, and quite differ- 

 ent materials were drifted in afterwards, this question is not diffi- 

 cult to decide ; for then the impression extends very little distance 

 up or down, and is quite imperfect, save on one layer, which of 

 course will be regarded as the one originally trodden upon. And for- 

 tunately such is the case with the larger proportion of tracks. But 

 where the materials were very soft, it would seem as if the toes 

 sank considerably into the mud, and were withdrawn without much 

 disturbance ; though afterwards the edges of the impression thus 

 made approached each other. In no other way can we explain the 

 extreme narrowness of some of the tracks found on the fine red shale, 

 of Wethersfield especially. There, as already remarked, the impres- 

 sions sometimes extend through from one to four inches, and the lay- 

 ers are bent down so as to be almost perpendicular to the surface. 

 Some have thought that in this case we could determine how far 

 the animal sank, by finding where the depressed laminae of rock 



