12 PLATYCRINID.E.^PLATYCRINITES. 



joints, which are also thicker and thinner alternately. The articulating facets of those 

 joints which are ciixular arc fiu-rowed with diverging striae, the elevated ridges of one 

 joint fittmg into corresponding depressions in those wliich adjom it. This structure 

 becomes gradually modified, and fm-nishes an instructive and mtcresting example of the 

 manner in which nature, ever fertile in expedients, varies the ordinary structure of parts 

 to meet the physical condition and wants in the economy of animals ; for whenever 

 circumstances require an altered mechanism, the amount of alteration necessaiy is 

 immediately produced with a certainty equally admirable for beauty of appearance, as it 

 is for its perfect adaptation to the purpose required. 



It has been already observed that at and near the dorso-central plate, the mode of 

 articulation between the columnar joints is carried on by numerous radii. At a short 

 distance do-^vn the column this structure becomes modified, the joints, by almost insen- 

 sible degrees, assume an elliptical form, and the diverging striae gradually become less 

 distinctly marked, while a transverse ridge is now for the first time famtly exhibited, 

 dividing the sm-faces of the joints into two equal parts. This elevated ridge becomes, 

 at each succeeding jomt, more and more strongly marked, while the marginal radii 

 become gradually fiiinter and fainter until they totally disappear. In exact conformity 

 Avith the change in the articidating fiicets of the joints, the outward form of the column 

 becomes gradually modified to suit the altered mechanism. As the radii become less 

 decidedly developed, the outward form of the joints gradually assume an eUiptical shape, 

 until at the precise point where the articulations between the joints are whoUy secured 

 by transverse ridges, the column becomes of a decidedly eUiptical form, the radiating 

 striae entirely disappear, the joints are no longer unequal in diameter or thickness, but 

 become uniform ui size and structure. 



The transverse ridges on the superior and inferior siufaces of one joint appear to have 

 a tendency to assume an oblique direction to the ridges of the adjoining joints. This 

 circumstance led Miller to suppose, that in a living state the superior and inferior 

 surfaces of each joint, with their articidating ridges, were alone truly calcareous, and 

 that a yielding muscular substance was interposed between them. After a very careful 

 exammation of many columns, we are miable to discover the least trace of such vascu- 

 larity as Miller imagined to exist. That the columns were really vascidar there can be 

 no reasonable doubt, but they were so exactly in the opposite direction to that in wMch 

 Miller believed them to be. He supposed the muscidar or cartilaginous substance 

 occupied a transverse space in each joint, instead of the numerous tubes which run 

 parallel to the axis of the column. 



We have reason to suppose that the column throughout its whole length was vascular, 

 or rather that the animal deposited calcareous matter witliin itself to give stability to its 

 pedicle, without destroying its flexibility. That, in fact, each joint of the column is 

 composed of irregular calcareous tubes, so that a transverse section exhibits somewhat 



