418 SMITHSONIAN INSTITUTION 



Onychocrinus is the first genus among the Taxocrinoidea in which the true heter- 

 otomous arm structure became thoroughly developed. Its characteristic feature, giving to 

 its typical forms a habitus wholly distinct from that of any other genus of the Flexibilia, 

 is the division of the ray into two free trunks or rami, on which the arms are borne as 

 subordinate, branching ramules, along with the extension of the main ray by the lateral 

 union and interlocking of one or more pairs of secundibrachs, so reduced in size that the 

 two together prolong the ray for some distance above the axillary at about the same size as 

 below. The extent of this union is a matter of age in the individual ; in young specimens 

 the interlocking of the secundibrachs has not begun, while in the mature forms of some 

 species it extends to five pairs of brachials. The genus is a direct development from 

 Taxocrinus (analogous to that of Barycrinus from Cyathocrinus), having the anal charac- 

 teristics of that genus carried to the extreme ; and with a very young specimen it is difficult 

 to distinguish between the two genera unless enough of the first ray division is visible to 

 show whether it forks again by equal bifurcation ; if not, it cannot be Taxocrinus. 



Along with this character is a tendency to increase in the number of primibrachs, which 

 in certain species gives an added length to the already long, free rays, and as said by Meek 

 and Worthen (Geol. Surv. Illinois, vol. 2, p. 243) " imparts a peculiar physiognomy to the 

 whole fossil quite obvious even to the most careless observer." These authors, at the place 

 cited, gave a most excellent discussion of the characters of the genus, and especially pointed 

 out that along with its free rays, it has the anal tube of the typical Taxocrinus. In the 

 extreme species, where the arms are borne in small clusters near the extremity of the free 

 rays, there is also a strong tendency for the whole crown to assume an extended position, 

 with the rays projecting horizontally from a rather small central disk, suggesting to those 

 authors comparison with a starfish or an Ophiuran, especially with the existing genus 

 Astrophyton. This resemblance had led Lyon and Casseday, the authors of the genus, to 

 the opinion that it forms a connecting link between the Crinoidea and Asteroidea ; a view 

 which Meek and Worthen, in the discussion above cited, showed to be without good foun- 

 dation — " the resemblance being merely simulative, and not due to any close analogy of 

 structure." 



The greatest increase in the number of primibrachs is seen in one of the British species, 

 0. polydactylus, in which they amount to 6 or 7. The first American species has only 3, 

 after which follows a strong line of species with 4, being all of the extreme form ; then the 

 structure returns to the original 3, with which the genus ends.- These differences are not 

 irregular, but hold good for the species in America with few exceptions. 



We have been able in Onychocrinus to study the ventral structure with unusual 

 thoroughness in two of the species, and to demonstrate for the second time among the 

 Flexibilia by perfect specimens the existence of the open mouth and pliant tegmen, by which 

 they differ so sharply from the other grand divisions of the crinoids. The genus represents 

 the extreme of the flexible characteristics on which the order is based ; the lower calyx 

 plates are rather thin, while those of the free rays are very deep and strong; but the whole 

 crown in some of the species could assume any position, from a complete horizontal disten- 

 sion of calyx and rays to one with the rays vertical and closely folded. We find specimens 

 in all varieties of these positions, and on account of such extreme changes the usual measure- 

 ments are not of much service. A useful basis of comparison is found in the relative length 

 of the ray below the fork, and of the main trunks or rami above it ; there is a very well- 

 marked progressive variation in regard to this character through a line of species, from 

 those with very short rami in the Burlington to some in the Keokuk and Kaskaskia with 

 rami two or three times as long as the ray below them. 



Lyon and Casseday proposed the genus upon the Keokuk species exsculptus, which 

 represents the extreme form so well described by Meek and Worthen. The former authors 

 did not include it in their species ramulosus, which they had previously described under 



