75 
Reference. 
PARKINsoN's Organic Remains, Vor. rr. t. 17. fig. 12. 
G. CumBERLAND, in Trans, of Geol. Soc. Vor. v. t. 5. fig. 8. 
Locality. 
In the Mountain Limestone of the Mendip Hills, the Black Rock (14th bed: 
of Dr. BRIeaTs Series in Geol. Trans. Vol. ıv.) near Bristol, Dublin, and 
Cork. 
Description. 
The CoLumnN (Pr. 11. fig. 1. 18. to 20.) of this animal is formed of numer- 
ous elliptical joints, which near the pelvis are of an almost round figure (Pr. 1. 
fig. 1. to 3.) and are for some distance from it placed alternately, so that a 
` larger and thicker joint succeeds a smaller and thinner. The articulation of 
the pelvis with the column in this species (Pr. 11. fig. 1. and 2.) affords an in- 
teresting example of the manner in which the ordinary structure of the parts of 
animals is changed, whenever the circumstances are such as to require a new 
mechanism ; for the mode of articulation between the joints throughout the 
rest of the column, is by means of a transverse ridge only, that mode 
being most fitted for their elliptical form; but since the upper columnar joint 
has to support a pelvis composed of three pieces, which could not, from their 
mode of construction, be affixed to a single ridge, this plan is here abandoned 
and a new one introduced, which consists in the joint assuming a round figure, 
having at its upper surface a margin marked with radiating strie, on which the 
inferior striated surfaces of the three pelvis plates are applied in the usual man- ` 
ner; and the transverse ridge is here very faintly exhibited. Similar marginal 
radii are seen, but more and more faintly, in several joints below (PL. 1. fig. 3. 
to 15.) and soon totally disappear, when the joints become of a true elliptical 
form and thicker. The transverse ridge on the superior and that on the inferior 
surface of the joint have (even when these are not thicker than a piece of writing“ 
paper) a decided tendency to assume a direction oblique with respect to each 
other, which is still more conspicuous when the joint is larger (Pr. 11. fig. 10. and 
11.) This appearance bas led me to suppose, that in a living state the inferior 
and superior surfaces of each joint with their articulating ridge, were alone truly 
calcareous, having a yielding muscular or cartilaginous substance inter posed 
between them. In consequence of the adhesion ot the superior surface of each 
