366 PALAEONTOLOGY OF ILLINOIS. 



proboscis. This mere peculiarity of form, however, might be of little 

 importance in a group presenting such great differences in this respect ; 

 but we observe that the arm-bases in the specimens of this type we 

 have seen are usually stouter and composed of rather wide short pieces, 

 more like those composing the arm-bases of Agaricocrinm. From this 

 fact \ve suspect that this type may present some marked differences in 

 the nature of its arms from the typical Batocrinus. 



The other group (Eretmocrinus) is mainly distinguished by a remark- 

 able flattening of the upper part of the arms, by which they are made 

 to present a very curious paddle-shaped or spatulate outline. In some in- 

 stances this character is so strongly marked, that the breadth of the arms 

 is not less than six times as great above as below the middle. Below, 

 the arms are, as in other types, usually rounded and slender, but farther 

 up the flattening commences, first, by a slight angularity along each 

 side, with often crenate margins, and increases upward above the mid- 

 dle until they sometimes present a very extraordinary alate appearance. 

 The flattened part, however, is always as distinctly composed of a 

 double series of alternately arranged pieces as that below, and these 

 pieces are not only extended laterally to give breadth to the arms, but 

 have also often as much as tAvice the diameter, in the direction of the 

 length of the arms, of those further down. The ambulacral furrows, 



however, do not increase in size with the breadth of the arms, but even 



7 7 / 



seem to be smaller above than below r . We have not seen tentacula 

 attached along the flattened upper part of the arms, but they probably 

 existed there, as we have observed minute indentations at the inner 

 ends of the flattened pieces, apparently for their attachment. The 

 pieces composing the flattened part of the arms are thicker at their 

 inner ends and thin off to their outer extremities, with slight outward 

 curve, so as often to make the dorsal side of the arms not merely flat, 

 but even slightly concave. 



If these were free Crinoids, we might suppose this flattening of the 

 arms a natural provision to adapt them for use as swimming organs, as 

 Comatula is known to employ its arms for that purpose ; but the species 

 presenting this character have the column as well developed as we see 

 in any of the other types, and were evidently attached to one spot du- 

 ring life. It is not improbable, however, that this peculiarity of the 

 arms may have been a provision for the protection of the ova in the 

 teutacuia (pinnulse), for, when these broad flattened arms were folded 

 together, they must have covered these delicate parts within as if by a 

 coat of mail. 



The species presenting the character of arms, described above, agree so 

 closely in other respects with the typical forms ofBatocrinn.^ that it is per- 

 haps not always possible to distinguish them from specimens with the 



