POTERIOCRINID.E EXTRACRINUS. 101 



pouch containing tlie (lit^estive organs it is found to be of consifieiable size. In the 

 specimen represented in Pi. 12, fig. 1, b, the plated integument inclosing the capacious 

 abdominal pouch has been so beautifully preserved during the process of petrifaction 

 that it retains the exact form it possessed in life, and we see it resting between the 

 rays somewhat in the manner of a ball when poised on the cup made to receive it. 



This specimen was originally in the collection of Mr. J. Johnson, but it is now in the 

 possession of the Rev. — .Jackson. The plated integument has been likewise well 

 preserved in one of the group represented in our first plate; a dark cross indicates 

 the figure. 



The Mouth. — In the centre of the plated integument before described is the moutli, 

 which was slightly protrusive but not proboscidiform. It was well fitted for sucking in 

 soft fleshy prey, and being surrounded by the rays.it was easy for them or the tentacnla to 

 pass objects of sustenance from every part of tiie circumference to the oral orifice. 



The Colujin. — The column is pentagonal and deeply sinuated between the salient 

 angles. The upper portion is composed of ossieula alternately thicker and thinner, 

 with a still thinner joint interposed between every two of the larger articulations. The 

 intervening thin joints are the rudimentary new ones, which are always introduced 

 at the upper portion of the column. In this manner the column became length- 

 ened, and as new joints were developed at the summit, others towards the base 

 attained their full size, so that as one articulation arrived at maturity, a new one was 

 introduced to keep up the required degree of flexure, and thus to compensate for, or 

 restore the loss of flexibility which would otherwise have taken place through the 

 conversion of a thin and probably yielding joint into a full sized and comparatively 

 rigid one. 



Towards the base the columnar articulations gradually become more equal in size, 

 until at length the approximation is all but complete, and all the joints are nearly' of 

 the same form and structure. In PI. 12, and the explanatory observations, this subject 

 will b> fjimd more particularly detailed. The lower joints are ornamented with rows 

 of tubercles which run transversely to the axis of the column. 



Near the summit the larger columnar joints are slightly rounded at their articulating 

 rims, and the articulating surface is alternately convex and concave, while the smaller 

 and thinner joints are partially compressed, but the angles to which the stellular or 

 floriform crenulations extend are doubly convex, that is, bulged out both above and 

 below. The small joints fit admirably into the depressions in the larger joints, while 

 the convex surfaces of the latter occupy the depressions in the smaller ones, so that 

 by each surface of articulation being alternately convex and concave an amount of 

 strength was secured which would defy all chance of dislocation by ordinary and 



