Fig. 1. A beautiful specimen of Plesiosaurus macrocephalus 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 Pl. 16, 17. the following differences are obvious:

l. The head is very much larger and longer, being nearly one half the length of the neck.

2. The vertebræ 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. dolichodeirus. 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 differences 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. Macrocephalus, than in the more slender limbs of P. Dolichodeirus.

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 Dohichodeirus, copied from the specimen figured in Pl. 17.

3. Anterior extremity of an insulated lower Jaw of Plesiosaurus, from the Lias at Lyme Regis, in the British Museum, part of the collection of Mr. Hawkins. V. I. p. 205. Note. (Original.)

4. The entire bone, of which Fig. 3. forns part, reduced to a small scale.