16 BRITISH FOSSIL REPTILES. 



lines is maintained through nearly the whole of that length, in corresponding* close 

 contact with the contiguous spines. In the thirty-seventh vertebra (Tab. VI [, 37) 

 the length of the neural spine is five and a half inches ; it has a little increased in thick- 

 ness ; the fore-and-aft diameter continues the same. In the second dorsal the neural 

 spine is six inches four lines in length, with a thickness of six lines. These dimensions 

 are continued to the fifty-eighth vertebra, save that, in the posterior half of the dorsal 

 series, the spines have less fore-and-aft breadth at their proximal third, and leave cor- 

 respondingly wider intervals ; they are in contact at their more expanded distal 

 portions. From the fifty-eighth vertebra they gradually decrease in length to the 

 sixty-second, or sacral vertebra, showing a height of less than four inches, with a ter- 

 minal fore-and-aft extent of two inches, and a thickness of six lines. They decrease 

 in all dimensions as the caudals recede from the trunk, and most so in fore-and-aft 

 extent, leaving wider interspaces ; by which character, with the higher position on the 

 centrum, and anchylosed condition of the pleurapophyses, a caudal vertebra may be 

 distinguished from a cervical of similar size. The caudal centrums are also thicker in 

 proportion to their length, and the under surface, if exposed, would doubtless also 

 yield the character of the hfEmapophysial pits. 



The dorsal diapophyses progressively increase from the first (Tab. VII, d 1, 2, d), and 

 attain, at the fifth dorsal vertebra (Tab. V, fig. 1), a length of two inches three lines 

 along the upper border. The rib-surface is cut from above downward and inward, 

 shortening the under extent of the process. A low ridge is continued from the pos- 

 terior angle of the neurapophysis upon the back part of the diapophysis, which expands 

 to the truncate articular surface. After the sixteenth dorsal the diapophyses gradu- 

 ally shorten to the sacral vertebrae, where they have almost subsided. 



The zygapophyses in the neck (Tab. V, fig. 2, z, z') and greater part of the back 

 are nearly horizontal, the anterior ones looking a little inward as well as upward, the 

 posterior ones the reverse ; they are given off nearer the base of the neurapophysis 

 tlian usual (compare Tab. V, fig. 2, -, y with Tab. Ill, fig. 4, Plesiosaurus dolichodeirus, 

 and Tab. X, fig. 1, PL rostratus), towards the end of the back their aspect gra- 

 dually changes; and, in the tail, the articular surface becomes almost vertical; 

 that of the anterior ones, which are most developed and longest retained in the 

 vertebral series, looking inward. The terminal articular surfaces of the centrum 

 of the last dislocated caudal vertebrse are, as usual, more concave than in the 

 neck. 



The development of the neural spines throughout the trunk and base of the neck 

 is such as to impede inflection in the vertical direction. At the anterior half of the 

 long and slender neck this bend would, indeed, take place in some degree ; but the 

 greatest flexibility would be from side to side. The provision for the attachment of 

 the vertebral muscles in the trunk is very great, indicative of corresponding power of 

 regulating the movements and position of the body during the application of the 



