OF FOSSIL FOOTMARKS. 257 



portion to the others than in O. g-racilior. It occurs on the gray- 

 micaceous slate of the Horse Race. 



Last winter, (1842,) I took the sketch of the ti-ack of a snoiv 

 bird {Fring-illa Hudsonia, I believe,) on snow, and on discover- 

 ing the O. Rogersi, I was struck with the resemblance between 

 them. This will be obvious by comparing figs. 7 and 8 ; both 

 of the natural size. The latter, however, is more delicate, and 

 the hind toe points somewhat more directly backwards than in 

 the fossil track. Nevertheless, it would be easy to imagine that 

 the same species of animal made them both. 



All the species of Ornithoidichnites which I have hitherto de- 

 scribed, I have been able to bring under two general divisions ; 

 the Leptodactyli, or slender-toed, and the Pachydactyli, or 

 thick-toed. But the two next species, which I have to propose, 

 appear to me to belong to neither of these divisions ; and to re- 

 quire a new term to express their general character. I propose 

 to call them Pterodactyli, or iving'-toed; because they appear 

 to have been made by an animal, which, like the coot and the 

 grebe, had a membrane, with an indented margin, extending 

 along its toes. The first species, which I shall describe, might 

 easily be mistaken for the O. tuberosus, when upon a surface 

 where the impression of the membrane would not be noticed. 

 But in the very perfect specimens which I possess, that impres- 

 sion is perfectly distinct ; especially around the claws ; and the 

 bottom of the foot is sunk but very slightly into the mud ; so 

 that it is perfectly distinct from the tuberosus, and is a most beau- 

 tiful track. I take the liberty to derive its specific name from that 

 of Charles Lyell, Esq. of London : not to make him better known, 

 but as a tribute of respect for his eminent services in the cause 

 of geology ; and as commemorative of the pleasure I recently 

 experienced in conducting him to some of the localities of our 

 fossil footmarks. The characters of the species are the fol- 

 lowing : 



Ornithoidichnites LyeUii Toes three, all in front : straight, spread- 

 ing forty degrees : curved expansions On the inner toe, two : on the 

 middle toe, three: on the outer toe, four: all remarkably distinct, 

 and making but a shallow impression : indicating a foot nearly fiat 



