350 PROCEEDINGS OF THE ACADEMY OF 



sometimes produced into short spine -like projections, but even where they 

 assume the character of spines they never have the regularity of arrangement, 

 nor do they ever attain the length we often see in Dori/crinus and Amphora- 

 criiius. The species of Balocrinus also present a much greater diversity of 

 form than we see in Actinocrinus , since we find amongst them every variety of 

 shape, from globose to turbinate, biturbinate, pyriform, discoid, and even in 

 some rare aberrant types apparently belonging here, a conical or stelliform 

 outline. 



The species of this genus may be variously grouped to facilitate their study 

 into sections and subsections, based upon their diflerences of form, and other 

 more or less marked peculiarities, but for the present we merely propose to 

 give a general list of them, and to notice a few types that we have elsewhere 

 included in this group, but which we are now rather inclined to think may 

 yet be found to be entitled to more prominence than has generally been sup- 

 posed. These are the forms for which the names Alloprosallncrinus (= Co- 

 nocrinus of Troost's list) and Eretmocrinus, Lyon and Casseday, were proposed. 



The tirst of these we only know from specimens consisting of the body 

 without the arms or other parts. Its most striking peculiarity, so far as yet 

 known, consists in its remarkable conical form, the body being nearly or quite 

 flat below the arm-bases, which are at first directed out horizontallj' and then 

 curve up ; while the vault, which forms the whole visceral cavity, is produced 

 upward in a conical form, so as to pass rather gradually into the central or 

 sub-central tube, or so-called proboscis. This mere peculiarity of form, how- 

 ever, 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 Ayarkocrinus. From this 

 fact we suspect that this type may present some marked differences in the 

 nature of its arms from the typical Batocrinus. 



The other group i^Eretmocrinus) is mainly distinguished by a remarkable 

 liattening of the upi)er part of the arms, by which they are made to present a 

 very curious paddle-shaped or spatulate outline. In some instances 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 com- 

 mences, first, by a slight angularity along each side, with often crenate mar- 

 gins, and increases upward above the middle 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 jjieces as 

 that below, and these pieces are not only extended laterally to give breadth to 

 the arms, but have also often as much as twice 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 seem 

 to be smaller above than below. We have not teen 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 jtart 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 during life. It is not improba- 

 ble, however, that this peculiarity of the arms may have been a provision for 

 the protection of the ova in the tentacula (jjinnulae), for, when these broad flat- 

 tened arms were folded together, they must have covered these delicate parts 

 within as if by a coat of mail. 



[Dec. 



