Structure of the Plesiosaurus Macrocephalus. 52^ 



Such is the general character of the spinal column as regards the compo- 

 sition of the vertebrae in the genus Plesiosaurus ; and consequently so far 

 the Plesiosaurus Macrocephalus agrees with the better known species : it 

 also presents the same generic peculiarities in the form of the articular sur- 

 faces of the bodies of the vertebrae as are described by Mr Conybeare in the 

 PL Dolichodeirus. 



I now proceed to describe the specific characters distinguishable in Lord 

 Cole's Plesiosaurus Macrocephalus. 



Cervical vertebra. (PI. XLIV.) 



The cervical region of the spine in this species exhibits the prominent character of the genus 

 in its great extension. It is, however, only twice the length of the lower jaw, instead of three 

 times the length of the same part, as in the Plesiosaurus Hawkinsii ; and this difference, arising 

 from the greater development of the head in the Macrocephalus, is associated, Dr. Buckland 

 observes*, with a thicker and stronger character of vertebrae in relation to the greater weight 

 they had to sustain. It includes 29 cervical vertebrae. 



In the 20th cervical vertebra of Plesiosaurus Hawkinsii the transverse is to the antero-posterior 

 diameter as 4 to 3. In the corresponding vertebra of the Plesiosaurus Macrocephalus the trans- 

 verse is to the antero-posterior diameter very nearly as 2 to 1. The rest of the cervical vertebrae 

 bear a similar ratio to those of the Plesiosaurus Harvkinsii ; the bodies of the vertebrae therefore 

 in Plesiosaurus Macrocephalus, although by no means so flat as in the Ichthyosauri, make an evident 

 approach to the characteristic form of the vertebrae in that genus. 



Mr. Conybeare has justly remarked how difficult it is to determine the number of the cervical 

 vertebrae in a Plesiosaur, owing to the gradual transition in their lateral appendages from the con- 

 dition of hatchet-shaped laminas to the ordinary elongated form of ribs. 



For the purpose of gaining a point of comparison of the different species of Plesiosauri, I 

 would suggest that the vertebra in which the costal articular surface has first entirely left the 

 centrum and passed upon the neurapophysis should be reckoned as the first dorsal. According 

 to this criterion, the number of cervical vertebrae in the Plesiosaurus Hawkinsii will amount to 31, 

 while those o( Plesiosaurus Macrocephalus are 29. 



In the Plesiosaurus Hawkinsii, the hatchet-shaped processes are converted into styliform ribs 

 at the 29th cervical vertebra ; but in the Plesiosaurus Macrocephalus they undergo this change 

 of form at the 27th cervical vertebra, and perhaps at the 25th, but this appendage is lost in the 

 skeleton under consideration. 



Hence we may conclude that the PI. Macrocephalus has two vertebrae less in the cervical re- 

 gion than the Plesiosaurus Hawkinsii, and probably six cervical vertebrae less than the PI. Doli- 

 chodeirus, in which Mr. Conybeare states that " the thirty-five anterior vertebrae exhibit these 

 (hatchet) processes distinctly characterized, and are therefore beyond all doubt cervicalf." 



The articular surfaces for the ribs on the anterior cervical vertebrae of the Macrocephalus are rela- 

 tively larger and have a more regular lozenge shape than in the Plesiosaurus Hamkinsii,m'wh\c\\ they 

 are elongated in the axis of the vertebra. They are traversed (as mentioned in the general characters 

 of the Plesiosaurian vertebrae) by a longitudinal groove ; this gradually sinks from the middle of 

 the depression towards its lower margin, and at length, at the 23rd cervical vertebra, disappears. 



* Bridgewater Treatise, ii. p. 30. t Geological Transactions, i. (Second Series) p. 384. 



VOL. V. SECOND SERIES. 3 Y 



