Structure of the Plesiosaurus Macrocephalus. 525 



In adult individuals of the Plesiosaurus Macrocephalus, these separate elements of the supe- 

 rior arch become anchylosed together, as is the case in a great part of the spine in the present 

 specimen. 



In a Plesiosaurian cervical vertebra, however, measuring seven inches and a half in vertical 

 extent, and three inches and a half in transverse diameter, in the collection of Mr. Hawkins, I 

 find the neurapophyses distinct both from the spine above and the centrum below. But in other 

 cervical vertebraj of a still larger Plesiosaur in the collection of Lord Cole, not only is the spine 

 anchylosed with the neurapophyses, but these are also confluent with the centrum. 



In the dorsal region in the Plesiosaurus Macrocephalus, as in the PL Hawkinsii and PI. Doli- 

 chodeirus, the neurapophyses and spines become anchylosed ; but the former elements continue 

 separate from the body of the vertebra throughout the vertebral column in the Macrocephalus. 



The cervical spines in the PL Macrocephalus differ in form from those of Plesiosaurus Haw- 

 kinsii, in retaining their breadth or antero-posterior extent throughout the neck ; their extremi- 

 ties being, as it were, truncate, with the angles slightly rounded off. The powerful ridge of bone 

 which they thus collectively form, is highly characteristic of this species. The consequence of 

 this structure is a diminution of the spinal interspaces necessary for the vertical inflections of the 

 neck ; which interspaces are conspicuously present in the Plesiosaurus Hawkinsii, where the end 

 of each cervical spine is, as it were, obliquely cut off at the anterior part, so as to allow the neck to 

 be bent upwards much more extensively than could have been possible in the Macrocephalus *. 

 What, however, the latter species thus lost in mobility, it gained in strength, the quality mainly 

 required in relation to the movements of its more bulky head and jaws. 



As the powerful neck of the PL Macrocephalus, however, possessed extensive mobility in the 

 lateral direction, as is indicated by its position in the fossil, the muscles destined for these move- 

 ments must necessarily have been developed in a corresponding degree ; and we find that adequate 

 provision was made for their fixed points of action, in the superior development of the costal pro- 

 cesses, as compared with those of PL Hawkinsii : — these processes present, indeed, throughout a 

 greater part of the neck the characteristic expansion of their distal extremities, which led to their 

 being called hatchet-shaped bones by Mr. Conybeare : but the stem which supports the dilated 

 extremity is proportionally longer in the Macrocephalus ; and it is only towards the base of the 

 neck that the extremities overlap each other, as in the Crocodile. Dr. Buckland has illustrated 

 this peculiarity by placing side by side the figures of the hatchet bones in the PL Hawkinsii and 

 PL Macrocephalus. These cervical ribs assume the true costal form, as before stated, at the 

 27th vertebra, where they are short and straight ; behind this part they progressively increase in 

 length, and become bent towards the sternal region. 



The cervical vertebrae gradually increase in all their dimensions, (least so, however, in their 

 antero-posterior extent), as they approach the trunk ; but the difference in their size at the two 

 extremities of the neck is less than in the Plesiosaurus DoUchodeirus. 



Dorsal Vertebra. 



These are characterised, agreeably with the previous definition, by the absence of articular sur- 

 faces for the ribs at the sides of the centrum, and by the development of a superior transverse 

 process (exclusively supporting the rib) from the base of each neurapophysis. 



* In the large posterior cervical vertebrae of the Plesiosaurus in Mr. Hawkins's collection, the 

 extremities of the spines are expanded ; and the anterior obliquely-truncated surface is flattened. 



3y2 



