rOTEIUOCRINID.i:. — POTERIOCRINUS. 71 



with the attenuated body below the rays, form one of the most interesting characters 

 not only of the species, but of the genus. 



In the P.crassns the oral tube attained the lengtli of four or five inches, and two inches 

 and a half in circumference, and it appears to have been well fitted to gorge prey of 

 larger size than most other crinoids. 



The oral tube has been frequently found detached from tlie body, and free from the 

 matrix in which it had been imbedded, on the shores of the Bristol Channel, at Clevedon, 

 where it is mostly worn by the abrading action of the salt water, but still retaining suffi- 

 cient evidence of its structure to enable us to indentify it with certainty as belonguig to 

 the P. crassus. 



In its detached state it has often been mistaken for a gigantic fossil leech, or worm. 

 — Figure 1. Plate 9, is from a specimen in the collection of Miss Rich who liberally 

 granted us the use of the crimoids in her cabinet. 



The Rays. — The primary rays are five, each composed of several joints or ossicula. 

 Each ray articulates by grooves and ridges on the basal surface of its lower joint, which 

 grooves and ridges fit into corresponding elevations and depressions on the upper 

 edge of the ray bearing plate. The ridges are perforated for the passage of the muscles 

 which held the rays in their assigned position, and at the same time imparted motion 

 to them. 



The first bifurcations increase the number of rays to ten, the second amount to 

 twenty, but the specmiens examined do not exhibit further divisions. 



The rays are deeply sulcated inwardly for the passage of the muscles, wliich were 

 protected by an integument covered with minute plates. 



All the rays, the minor digitations, as well as the primary rays, are composed of single 

 series of joints, consequently as each joints only bears a single tentacula, the latter are 

 less numerous and wider apart than in those crinoids whose rays are composed of a double 

 series of joints. 



The ra}'s of the P. crassus are long and of great strength when compared with the 

 body. 



The Column. — ^The column is circular and composed of numerous thin joints, which 

 articulate by the radiating strire on each of their facets. Towards the upper portion 

 of the column the joints gradually decrease in thickness, and as gradually enlarge in 

 diameter until they form a surface equal in size to the lower portion of the crinoid's body. 



The central canal running through the column is pentagonal in its upper portion, and 

 circular as it approaches the base. It also becomes larger as it recedes from the body. 



