76 POTERIOCRINID^. — POTERIOCRINUS. 



The P. roslratus is smaller than the typical species, and the plates inclosing the 

 digestive organs are also much thinner; the structure of the oral tube is likewise different. 

 In the typical species the plates which encircle the proboscis are closely striated or ridged 

 transversely, while in the rostratus some of the ridges cross the plates horizontally, and 

 others obliquely so as to impart a reticulated appearance to the organ. 



'I'he proboscis is composed of several vertical bands of plates which extend upwards 

 from the base of the rays to the height, in a full grown specimen of two or three inches, 

 and terminates in several spiniform points. 



The tooth like plates at the apex of the oral tube are arranged in a manner which 

 would have enabled the animal to use them either as instruments for picking up and 

 holding the objects of its sustenance, or for the purpose of reducing them to a size, 

 shape and consistency fitted for gorging. The pointed plates of the oral tube, may in 

 some measure be compared to the dental apparatus, or Aristotle's lantern as it is some- 

 times termed, of the recent sea urcliins. And the structure of this organ is another proof 

 if proofs were wanting, that the office of the lily-stars was, as is that of the recent echini 

 and astericB, to check the too great increase and keep within bounds certain creatures by 

 feeding on them or their ova, while in turn the crinoids were doomed to support, in part, 

 animals more perfect in organization than themselves. 



We have developed several specimens of this Poterioorinus in different states of preserva- 

 tion, three of which with a considerable portion of the rays and columns attached. Two 

 of the immber also shew the lower portion of the oral tube sufficiently defined to prove 

 its identity with the more perfect though detached specimen of proboscis represented 

 in plate 9. figure 2. d. 



This beautiful species of Poterioorinus was first discovered by the Authors in the moun- 

 tain limestone of Ireland, since which they have ascertained its occurrence in the same 

 formation on the banks of the Avon, Bristol; as also in the magnesian beds of the 

 mountain limestone atClevedon, where Miss A.Rich was so fortunate as to develope the 

 beautiful column which forms so striking an object among our illustrations. 



In this specimen the body appears to have been bent round and acro.'s the column a short 

 distance below the summit, so that it hung down in the manner of a drooping flower. 

 Unfortunately only a part of the body has been preserved, but the impression made by 

 it is deeply indented on the surface of the rock in which it was imbedded. 



The auxiliary side arras are in this instance so perfectly preserved, that they are unbroken 

 to their extreme points, and the manner in which they increased in number is clearly 

 exemplified by the smaller arms in their early stage of growth emanating from the column, 

 above the larger and more perfect claspers below. 



As the side arms seldom occur above the lower third portion of the column, it follows 

 as a natural consequence that when newjolnts are formed by which its length is increased. 



