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 in- 

 creased ; 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 XIIL 

 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 kngth of the vertebral column but little can be said deci- 

 dedly, since no instance has yet been met with, in which both the na- 

 tural 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 num- 

 bers 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 considera^ 

 ble 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; 

 lias 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 



