84 POTEEIOCRINID^.— POTERIOCEINUS. 



Formation and Localities. 



Mountain Limestone. — Mendip Hills ; Black Rock, Avon Side, Bristol^ Barry Island. 



In the 5th vol. of the Transactions of the Geol. Soc. of London, at page 380, Mr. 

 Cumberland, asserts that he has found portions of the arms of this species in the Severn 

 Lias. We conceive this to be a mistake as to identity, for the genus had undoubtedly 

 become extinct before the age of the Lias. 



The upper part of the column of this species, is composed of alternate thick and thin 

 joints, but which probably become nearly equal ia size, at a short distance from the body, 

 a form of structure, very frequently observed in the crinoids hitherto examined. 



The number and arrangement of the rays of the P. tenuis, correspond with those of 

 the P. rostratm, but the joints of which they are composed, are comparatively much 

 longer. 



The P. tenuis is a small and delicate species, few individuals ever having attained the 

 size of Miller's figure at page 71, of his work on the Crinoidea. 



Miller has unfortunately represented the main rays, (hands) as articulating into small 

 excavations on the ray bearing plates, when in fact they are as wide at the points of 

 articulation as the plates themselves, which are not excavated, but ridged the whole 

 width as in most of the genus. He also represents the first joints of the secondary 

 rays (fingers) as "adhering near the base, laterally to each other." Instead of which 

 they fit on to the sloping edges of the cuneiform summits of the primary rays. This is 

 evident in the specimen from which our figures are taken, and also in another well 

 preserved specimen in our cabinet. 



The figure of this species in plate 10. is imbedded with the intermediate plates between 

 the rays at the open point of the circumference, and at the base of the proboscis, outward. 

 It was not however considered necessary to give a second figure in a different position 

 as the plates surrounding the body agree in number and arrangement with others of the 

 genus. 



Mr. Cumberland, who has given a figure of this species, in the Transactions of the 

 Geological Society of London, under the title of " a new and peculiar species of Encri- 

 nus," suggests the probability of the proboscis, which has been bent round and turned 

 outwards through the rays, above the intermediate plates, at its base, being an extraneous 

 body. After comparing this supposed extraneous body as represented in PI. 10, fig. 5, 



