350 PALEONTOLOGY OF ILLINOIS. 



cations, but perhaps not quite so marked, take place in Granatocri- 

 nus Norwoodi 0. & Sbum., and in Scliizoblastus (Granatocrinus] Sayi 

 Shum. 



Pentrcmites Godoni De Frame, in its earlier stages, is pyriform, 

 and resembles P. pyrifonnis Say, later on it is globose. The lower 

 portions, from the basals to the radial lips, are broadly turbinate 

 and decidedly longer than the summit portions. Afterwards they 

 become almost horizontal, and occupy, in large specimens, more 

 than three-fourths the height of the body, at a time when the am- 

 bulacra, which at first were scarcely longer than wide, attain a length 

 of more than three times their greatest width. 



The modifications which here take place in the basals and radials 

 are mainly produced by the increase in the length of the ambulacra. 

 These plates, and particularly the basals, had acquired already at 

 an early age a comparatively large size ; later on the body of the 

 radials increased much less in length than in width, as shown by 

 the lines of growth, which are sometimes exposed. The basals, how- 

 ever, which had attained almost their full height, and now had to 

 accommodate themselves to the increasing width of the radials, bend 

 outward, producing thereby the angularity at the outer side of the 

 radial cup, by leaving the lower thickened portions, which were less 

 pliable, in their former position. This explains fully the case as we 

 find it in Codonites, Cadaster, etc., in Pcntremites, however, under 

 similar conditions additional modifications have taken place. 



Restricting the genus Pentremites to species with large petaloid 

 ambulacra, most of them have at the lower end, at the junction with 

 the column, a little projection in form of a cone, which is almost as 

 prominent in small specimens as in the larger ones. This cone con- 

 stitutes the lower part of what appears to be a tri-partite plate, in 

 form of a Clover leaf, occupying the central portion of the basal 

 disc, into which it extends for some distance, following its curva- 

 ture. It is frequently somewhat elevated above the general surface 

 of the basals, and can be observed in most specimens with the 

 naked eye. The sutures which separate the basals by their shorter 

 sides in the usual way, divide the lower leaves lengthwise. From 

 external appearance, one feels very much inclined to take the inner 

 part of the basals to be an independent series of plates, but on 

 grinding the surface there is no intermediate suture. In one of my 

 specimens, which 1 take to be an extremely large specimen of Pen- 

 tremites Godoni, I find within the clover-leaf another leaf-like struc- 

 ture, but of less width, and beneath it eight joints of the column, 



