8o DAVENPORT ACADEMY OF NATURAL SCIENCES. 



equalized by the increase of the body in width, by which the ambula- 

 cra attain a greater curvature, pushing the basals, and ])artly the radi- 

 als, from a fairly sloping ])osition to a more horizontal one, as shown 

 in the following species, of which I have examined a large number of 

 specimens in all stages of growth. 



In the young Orophocrinus sielliformis the ambulacra occui>y only 

 the upper truncate side of the body, the lower portions are turbinate 

 with nearly straight sides; in very old specimens, however, the ambu- 

 lacra curve so strongl)', and reach down so deeply, that the radial lii)s 

 were brought into a horizontal i)osition, almost to the level of the 

 basals, and the sides of the body became concave, thereby pushing the 

 upper portions of the radials in a more outward direction. 



Schizoblasius ( Granatocriniis) vielo O. & Shum. is in its younger 

 stages elongate-ovate, in medium sized specimens subglobose to glo- 

 bose, and in large specimens depressed globose. The same modifica- 

 tions, but perhaps not tjuite so marked, take jilace in Granatocriniis 

 Norwoodi O. & Shum. and in Schizoblastus (Granatocriniis ) Sayi 

 Shum. 



Pentremites Godoni De France in its earlier stages is pyriform, and 

 resembles P. pyriform is Say; later on it is globose. The lower por- 

 tions, from the basals to the radial lips, are broadly turbinate and de- 

 cidedly 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 ambulacra, 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 accpiired 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, however, which 

 had attained almost their full height, and now had to accommodate 

 themselves to the increasing width of the radials, bend outward, pro- 

 ducing thereby the angularity at the outer side of the radial cup, by 

 leaving the lower thickened portions, which were less jjliable, in their 

 former position. This explains fully the case as we find it in Codonites, 

 Codaster, etc. ; in Pentremites, however, under similar conditions addi- 

 tional modifications have taken place. 



Restricting the genus Pentremites to species with large pjtaloid am- 



