30 EXPLANATION OF PLATE 19. 



Plate 19. V. I. p. 160. 



Fig. 1. A beautiful specimen of Plesiosaurus macroce- 

 phalus hitherto undescribed, found in the Lias marl 

 at Lyme Regis, by Miss Aiming, and now in the col- 

 lection of Lord Cole. (Original.) 

 On comparing this figure with those of P. Dolichodeirus 

 at PI. 16, 17. The following differences are obvious : 



1. The head is very much larger and longer, being nearly 

 one-half the length of the neck. 



2. The vertebrae of the neck are thicker and stronger in 

 proportion to the greater weight they had to sustain. 



3. The hatchet-shaped bones differ in form and size, as 

 may be seen by comparing them with those of P. dolicho- 

 deirus. PI. 19. Fig. 2. and PI. 17. 



4. The bones of the arm and thigh are shorter and 

 stronger than in P. Dolichodeirus, and corresponding dif- 

 ferences may be traced throughout the smaller bones of the 

 Paddles; the general adjustment of all the proportions 

 being calculated to produce greater strength in the P. Ma- 

 crocephalus, than in the more slender limbs of P. Dolicho- 

 deirus. 



These differences are not the effect of age ; as the two 

 specimens, from which they are here described, are nearly 

 of the same length. 



Fig. 2. Hatchet-shaped bones of the neck of Plesiosaurus 

 Dolichodeirus, copied from the specimen figured in 

 PI. 17. 



3. Anterior extremity of an insulated lower Jaw of Ple- 



siosaurus, from the Lias at Lyme Regis, in the Bri- 

 tish Museum, part of the collection of Mr. Hawkins. 

 V. I. p. 160. Note. (Original.) 



4. The entire bone, of which Fig. 3. forms part, reduced 



to a small scale. 



