30 EXPLANATION OF PLATE 19. 



Plate 19. V. I. p. 205. 

 Fig. 1. A beautiful specimen of Plesiosaurus macroce- 

 phalus hitherto undescribed, found in the Lias marl 

 at Lyme Regis by Miss Anning, and now in the 

 collection 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 larg;er 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. PL 19. Fig. 2. and PL 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 

 PL 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. 205. Note. (Original.) 



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



to a small scale. 



