150 AN ATTEMPT TO DESCRIBE THE ANIMALS THAT MADE 



indices of different species. In many species of birds, the hind toe 

 is simply the outer toe prolonged backwards, bringing the fourth 

 toe (pouce of the French) always on the inside of the foot. And 

 this is its situation in the fossil tracks ; as in the Ornithopiis Adam- 

 sonus, gallinaceus, gracilior, and loripes, Plate 8, figs. 1 - 4. In 

 the Plectropus minitans and longipes it is short, and proceeds from 

 a long heel, a little behind the origin of the toes, at right angles 

 nearly to the heel, like the spur of the domestic cock. Plate 8, 

 fig. 4, and Plate 9, fig. 3. In the Tricenopus Baileyanus (Plate 

 10, fig. 4), it is very slender, proceeding from about the same place 

 on a long heel, but directed forwards, so as to make quite an 

 acute angle with the heel. In the Tricenopus Emmonsianus (Plate 

 10, fig. 5), it proceeds from the end of the heel, and is directed 

 somewhat backwards, so as to form with the heel on the anterior 

 side an obtuse angle. In the Polemarchus gigas (Plate 9, fig. 1), 

 this toe, which is quite stout, proceeds laterally from a very thick, 

 rounded heel, at right angles to the axis of the foot. When this 

 toe runs directly backward, it is difficult to distinguish it from a 

 narrow heel ; as in the Macropterna rhjnchosauroidea, Plate 15, 

 fig. 9. In this case I have indeed considered this projection as a 

 heel, as the generic name implies. But the track of the snow- 

 bird (Fringilla Hudsonia) is almost exactly like fig. 9, except the 

 short outer toe ; and it is a hind toe that makes the posterior im- 

 pression. (See Transactions of the Association of American Geol- 

 ogists and Naturalists, Plate 11, fig. 8.) 



In dissecting some specimens of Plectropus, I have been struck 

 with another fact. On the highest layer the fourth toe appears to 

 project at right angles with the heel, and some distance back from 

 the roots of the other toes. But a little farther down we find its 

 extremity turned backward, and its other end forward, until at 



