LIASSIC PLESIOSAURS. 25 



summit, which is bent backward. The dorsal vertebrae continue to increase in the 

 length and size of the diapophyses, in the height of the neural spines, in the breadth 

 and depth of the centrum, and, by a still greater degree, in the length of the ribs ; in 

 every dimension, in short, except that of the length of the centrum, which, in the 

 tenth dorsal, is 1 inch 10 lines, and in no dorsal vertebra exceeds 3 inches. The 

 breadth of the centrum in the tenth dorsal is 3 inches ; the height, 2 inches 3 lines ; 

 the articular surface is moderately hollow at the middle, and gently convex towards 

 the peripher)^ ; the neural spines gradually attain the height of 3^ inches towards the 

 end of the series, the fore-and-aft extent being about I3 inch near the summit, which 

 is more thickened and truncate than in the neck, measuring, in some of these vertebrae, 

 9 lines in thickness. Both margins are concave at the lower half of the spine, and 

 the intervals between those of different dorsal vertebrae average about three fourths 

 of an inch at the narrower parts of the spines. The length of the diapophysis 

 is about 1^ inch; it expands to its extremity, which is abruptly truncate, looking 

 obliquely outward, backward, and a little downward ; it is flat, and rather rough, for 

 ligamentous union with the rib ; suljquadratc in form, averaging about an inch across. 

 The ribs attain their greatest length from the twelfth to the fifteenth dorsal, where 

 they are 1 foot 6 inches in length, with a simple expanded end, corresponding in shape 

 and size with the diapophysial surface ; the body of the rib is subcylindrical, then sub- 

 trihedral, and again subcylindrical in shape, about 6 lines in diameter at the narrower 

 part, and gradually enlarging at the distal third to the truncate extremity, which was 

 ligamentously connected with the sternal rib. Some of the longest ribs have suffered 

 fracture, and some contortion at their middle slender part, in the course of the 

 cosraical pressure which has spread them out flat ; but they retain much of their natural 

 curvatures on each side the vertebral column. After the thirteenth, the ribs gradually 

 decrease to a length of 3^ inches, in the last vertebra, in which the rib articulates 

 wholly with the diapophysis (twenty-second dorsal), the breadth of this rib is 5 lines. 

 Where the rib begins again to descend from the centrum, it continues to decrease in 

 length in the first and second, in the latter of which it begins to gain in thickness. In tlie 

 forty-ninth vertebra, counting from the skull, which vertebra I have indicated (Tab. IX, 

 s) as the first sacral, the rib is 2 inches 6 lines in length, and 9 lines in least diameter ; 

 its head is partly buried in the matrix, but the articular surface next the vertebra from 

 which it is detached is 2 inches in vertical and I inch in longitudinal diameter, and 

 the surface projects below, from the centrum, as it does above, from the neural arch. 

 The borders of the terminal articular surfaces of the centrum are thicker and rougher 

 than those of the dorsal or caudal vertebrae, indicating a stronger connection between 

 the vertebrae from which the pelvic arch was suspended. The rib of the second sacral 

 is straight, 2| inches in length, and 13 lines in the smallest diameter. 



In the caudal vertebrae the neural spines gradually decrease in length, but more so 

 in antero-posterior breadth, being longer, and with wider intervals at the basal half of 



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