BRITISH FOSSIL REPTILES. 6^ 



but in the PL macrocephaliis they undergo this change of form 

 at the twenty-seventh cervical vertebra, and perhaps at the 

 twenty-fifth ; but this appendage is lost in the skeleton under 

 consideration. 



Hence we may conclude that the PL macrocephalus has two 

 vertebrae less in the cervical region than the PL Haivkinsii, 

 and probably six cervical vertebrae less than the PL dolicho- 

 deirus, in which Mr. Conybeare states that ^' the thirty-five an- 

 terior vertebrae exhibit these (hatchet) processes distinctly cha- 

 racterized, and are therefore, beyond all doubt, cervical*." 



The articular surfaces for the ribs on the anterior cervical 

 vertebrae of the PL macrocephalus are relatively larger and have a 

 more regular lozenge-shape than in the PL Haivkinsii, in which 

 they are elongated in the axis of the vertebra. They are tra- 

 versed (as mentioned in the general characters of the JPlesiosau- 

 rian vertebrae,) by a longitudinal groove ; this gradually sinks 

 from the middle of the depression towards its lower margin, 

 and at length, at the twenty-third cervical vertebra, disappears. 



The depressions above the costal surfaces for the lodgement of 

 the bases of the neurapophyses resemble in form those of the 

 PL Hawkinsii, but extend further down upon the side of the 

 centrum. They are co-extensive with the antero-posterior dia- 

 meter of tlie vertebral body, and are bounded by two lines meet- 

 ing below at a right angle. The angle formed by the corre- 

 sponding lines in the PL Haiukiiisii is more open. The di- 

 stance between these neurapophysial pits and the costal pits in 

 the anterior cervical vertebrae, differs in different species of Ple- 

 siosaiirus. In the PL macrocephalus the interspace is very short, 

 never exceeding half the diameter of the costal pit, even in the 

 most anterior of the costal vertebrae. In the PL Haivkinsii 

 the interspace is equal to double the diameter of the costal pit 

 in the corresponding vertebrae. 



There may also be observed in the PL macrocephalus an evi- 

 dent tendency in the surface supporting the cervical vertebrae to 

 rise above the level of the centrum ; and this is the more inter- 

 esting as in a large species of Plesiosauriis allied to PL ma- 

 crocephalus (the PL arcuatus subsequently to be characterized) 

 the surface on the centrum, and the corresponding surface of 

 the neurapophysis do project as short transverse processes, and 

 thus approximate to the Crocodilian type. 



As the seventh, eighth, and ninth vertebrae happen to be dis- 

 placed in Lord Cole's specimen, and their neurapophyses to be 

 dislocated, the form and breadth of the articuhu* depressions for 



* Loc. cit., p. 384. 



