322 [Dec. 18, 



for there are two dorsal vertebrae in the trivertebral plate, thirteen 

 follow it, and two lumbar are anchylosed with the sacral, making 

 altogether seventeen. 



The ' sacrum,' composed of anchylosed lumbar, proper sacral, and 

 coccygeal vertebrae, contains at fewest twelve, and perhaps thirteen 

 vertebrae. The centra of the two lumbar vertebrae and of the two 

 proper sacral vertebrae which follow them are preserved. They are 

 thin and broad plates, flat above and slightly concave below, exhibit- 

 ing a most marked contrast with the half- cylinder of the hindermost 

 of the thirteen dorsal vertebrae above described. It would seem to 

 require the interposition of at least two, if not three, vertebrae to 

 effect the transition of the one form of centrum into the other. 



The last coccygeal is the only vertebra among all those preserved 

 the centrum of which exhibits characfers at all like those of an or- 

 dinary mammal, its terminal face being a very broad oval, slightly 

 concave, disk. The centrum of the penultimate coccygeal is much 

 natter and narrower ; and this flattening and narrowing predominates 

 still more in the antepenultimate and that vertebra which lies before 

 it, or the fourth from the end. From this point to the two anterior 

 sacrals the floor of the vertebral canal is completely broken away, but 

 there can be no doubt that the centra were represented by a thin 

 bony plate. 



The line of the centra of the coccygeal vertebrae forms a very 

 marked arch behind the two sacral vertebrae, whose centra form a 

 nearly horizontal floor ; while the dorso-lumbar vertebrae (including 

 the trivertebral bone) form a second arch, flatter than the first. 



The spinous processes of all these lumbo-sacro-coccygeal vertebrae, 

 up to the fourth from the end inclusively, are anchylosed together 

 in a long and strong osseous crest, broad and extremely rugose above, 

 eight inches high in front, but slowly diminishing as it follows the 

 curve of the centra posteriorly to five inches. 



The spinous process of the penultimate coccygeal vertebra is very 

 thick, but is broken short off. It was probably not less than 4 inches 

 high, and aiforded a middle point of support for the carapace between 

 the ischial protuberances. The sides of the median crest, and of the 

 two vertebrae which appear to constitute the true sacrum, are anchy- 

 losed firmly with nearly the whole of the inner edge of the vast 

 ilium. Behind these the vertebrae seem to have been devoid of 



