176 
vertebrae, differing nearly alternately from each other, both in size and 
form : larger orbicular vertebrae having smaller cylindrical ones .placed 
between them, as at Plate XIII. Fig. 43 and 44, and at Plate XIV. Fig, 1, 
where a sketch in outline is given of the conformation of the vertebral 
column from another specimen. 
By this most curious and well adapted species of articulation, the 
external edge of the smaller vertebrae, articulating with the inner edge of 
those which include it, the mobility is very considerably increased ; since, 
instead of being limited to the depth of the crenated edge, it is only 
limited by the collars of the largest vertebrae coming into contact, whilst 
moving on the external surfaces of the smaller vertebrae which they 
encompass, as is seen in the specimen Plate XIII. Fig. 8, and has 
already been observed, whilst describing the trochitae Fig. 23 and 25. 
By this admirable contrivance, it seems, that the animal possessed the 
power of moving its superior parts with all the facility and freedom 
which its instinctive pursuits might require. 
Of the length of the vertebral column but little can be said decidedly, 
since no instance has yet been met with, in which both the natural 
terminations have appeared on the same specimen of the trunk, without 
which it is obvious that no positive judgment respecting this circumstance 
can be formed. Reasoning, however, from the vast numbers of the 
vertebrae, and from the very small number of the extremities of this 
animal which have been found, there appear to be sufficient grounds for 
imagining that the trunk must have been of considerable length indeed. 
In one of my specimens, that from which the sketch in outline, Plate XIV. 
was taken, the vertebral column, still united to the body of the animal, is 
nearly eleven inches in length. 
I shall now proceed to the examination of that part of the skeleton of 
this animal which formed its body and superior extremities, and which 
has been hitherto called the lily, the crown, the head, or the body 
part, Plate XIV. Fig. 3. I shall first examine the basin (pelvis), in 
