1846.] LYELL ON FOOT-MARKS IN THE COAL-MEASURES. 417 



P. macrocephalus^ Ow. 



The neck is as four to two of 

 the head. 



P. brachycephalus^ Ow.* 

 The head is equal in length to 



fourteen of its 

 vertebrae. 



anterior cervical 



P. macrourus^ Ow. 

 The anterior paddles are the 

 largest, as in Ichthyosaurus, 



P. arcuatus, Ow. 

 The vertical height of the cer- 

 vical vertebra is seven inches. 



The length of the humerus is 

 fourteen inches. 



Diameter of the expanded 

 symphysis of the lower jaw is 

 four inches. 



P. megacephalus. 



The neck is as three to two of 

 the head. 



The head is equal in length to 

 twenty of its anterior cervical 

 vertebrae. 



The anterior paddles are small- 

 est, as in most of the Plesiosauri. 



The vertical height of the cer- 

 vical vertebra is seven inches 

 eight tenths. 



The length of the humerus is 

 thirteen inches seven tenths. 



Diameter of the expanded 

 symphysis of the lower jaw is 

 five inches two tenths. 



Upon comparison with descriptions of the remaining species, there 

 are essential and especial differences which at once distinguish from 

 all those the known species hitherto discovered. 



DESCRIPTION OF PLATE XVIII. 



Fig. 1. Plesiosaurus megacephalus. Scale one inch = 1 foot. 



2. Ditto. View of the upper part of the anterior portion of the head. 



3. Ditto. Side view of the anterior portion of the head. Scale 2 inches 



= 1 foot. 



4. Ditto sterno-costal arcs removed to show the dorsal vertebrae. 



2. On Foot-marks discovered in the Coal-measures q/' Pennsyl- 

 vania. By Charles Lyell, Esq., F.G.S. 



1 intended to draw up a paper on what I have learned and ob- 

 served respecting the proofs alleged to have been found of the exist- 

 ence of mammalia, birds and reptiles in the Pennsylvania coal-field ; 

 but I cannot do this until I receive my specimens, and have had 

 more time to make sections and maps, in order to do the subject 

 justice. I must therefore content myself M'ith this brief notice. The 

 discoveries of the impressions to which I shall refer were made by 

 Dr. King, a physician of this place, whom I have seen, and who has 

 been most anxious to facilitate my investigations, and to give nie 

 every lielp in arriving at the truth, wliich was his only thought, 



* The skull is mutilated in the specimen on which this species was founded. 



