C6 REPORT — 1839. 



part they progressively increase in length, and become bent to- 

 wards the sternal region. 



The cervical vertebrae gradually increase in all their dimen- 

 sions (least so however in their antero-posterior extent) as they 

 approach the trunk ; but the difference in their size at the two 

 t^xtremities of the neck is less than in the PL dolichodeirus. 



Dorsal Vertebrce. — -These are characterized, according to 

 the previous definition, by the absence of articular surfaces on 

 the centrum for ribs, and by the development of a superior 

 transverse process, which exclusively supports the rib, from the 

 base of each neurapophysis. The number of vertebrae so cha- 

 racterized is twenty ; that of the corresponding vertebrae in the 

 PL Hawkinsil is twenty-three. 



These vertebrae include, besides the ordinary dorsal, those 

 which occupy the situation of the lumbar or ribless vertebrae in 

 the Crocodile ; but there are no such vertebrae in the trunk of 

 the Enaliosaurs, which in this respect agree with many Lacer- 

 tians. 



The special characteristic of the dorsal vertebrae in the PL 

 macrocephalus, as compared with the PL Haiv/einsii, consists 

 in their being, like the cervical, more flattened in the antero- 

 posterior direction and more concave at the sides ; in which 

 flatter particular they resemble the dorsal vertebrae in the PL 

 dolichodeirus more than those of the PL Hawkinsii. 



At the commencement of the dorsal series the lower margin 

 of the neurapophyses is angular, as in the neck ; but towards 

 the middle of the back they become rounded, and the articular 

 depressions in the body of the A^ertebra present a correspond- 

 ing form. The transverse processes progressively increase in 

 length towards the middle of the trunk, and again diminish as 

 they approach the tail ; the bases of the neurapophyses from 

 which they rise diminish in vertical extent in the same ratio, 

 and leave a greater proportion of the centrum free from their 

 embrace. The increasing length and upward inclination of the 

 transverse processes supporting the ribs, Mr. Conybeare has 

 justly observed, " seem intended to give a wider sweep to the 

 ribs," and relate to the acquisition of greater expansion of the 

 thoracic-abdominal cavity at the part where the largest viscera 

 were lodged. The spinous processes at the beginning of the 

 dorsal region diminish in antero-posterior extent, but slightly 

 increase in height ; they then increase in both dimensions to the 

 middle of the back, and thence gradually decrease to the tail. 



Sacral Fertehrcs. — There are no sacral vertebrae by anchylo- 

 sis in the Plesiosaurs. In Lord Cole's remarkable specimen of 



