75 



Reference. 



PARKINSON'S Organic Remains, VOL. n. t. 17. fig. 12. 



G. CUMBERLAND, in Trans, of Geol. Soc. VOL. v. t. 5. fig. 8. 



Locality. 



In the Mountain Limestone of the Mendip Hills, the Black Rock (14th bed 

 of Dr. BRIGHT'S Series in Geol. Trans. Vol. iv.) near Bristol, Dublin, and 

 Cork. 



Description. 



The COLUMN (PL. n. 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 (PL. n. 

 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 (Pt. n. 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 striiE, 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. n. 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 (Pi,, n. fig. 10. and 

 11.) This appearance has 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 interposed 

 between them. In consequence of the adhesion of the superior surface of each 



