46 SMITHSONIAN INSTITUTION 



equal bifurcation, which I call " dichotomous " in preference to " isotomous," 

 because we cannot confine the term to such equally and perfectly symmetrically 

 bifurcating rays as those of the Inadunate genus Cyathocrinus. There is a 

 tendency throughout the group to inequality in the branching of the arms be- 

 yond the first division, the interval between successive bifurcations being 

 longer on one ramus than on the other ; or one may become simple while the 

 other subdivides. So the term must be taken to mean simply that the rays divide 

 by more or less regular bifurcations, as opposed to those in which the branches 

 are given off in the form of lateral ramules markedly smaller than the main 

 trunk; these we call (2) " heterotomous." Under this class we may have 

 unilateral heterotomy, in which the relatively small ramules are restricted to 

 one side of the main ray division, usually toward the inside of the dichotom but 

 exceptionally in one species of Onychocrinus to the outer side; Dactylocrinus 

 and Wachsmuthicrimts are good examples of this type (Pis. XLI, XLIII). 

 Or the heterotomy may be bilateral, where the ramules are borne alternately 

 on opposite sides of the trunks, as in the typical Onychocrinus (PI. LXX). 

 The ramules may divide by nearly equal bifurcation, or by means of secondary 

 ramules. 



Between these two principal types there is some intergradation, so that in 

 regard to some forms it is hard to say to which they belong; thus we call 

 Sagenocrinns dichotomous, although there is not much difference in principle 

 between a specimen like that of Plate XTX, figure 2, and one of Lithocrinus 

 (PI. XX), which we call doubtfully heterotomous. The two types occur to- 

 gether in the Pentacrinites. Endoxocrinus and Teliocrinus are strictly 

 isotomous; Isocrinus is slightly heterotomous; Metacrinus more markedly 

 heterotomous; and Pentacrinus almost typically heterotomous. Bather in 

 subdividing the Flexibilia makes the heterotomous arm structure a family 

 character on which he bases his Dactylocrinidae — a group which includes a 

 representative of every one of the four families of my classification. I am 

 unable to attach the same importance to this character, as the two structures 

 occur together in several of the best defined families of the Camerata, as well 

 as in each of the families into which according to my view the Flexibilia are 

 divided. 



In one genus of the Lecanocrinidae, Cholocrinus, there is a remarkable 

 inequality in the development of arms in the different rays, the two posterior 

 rays and the anterior ray being of ample size, while the two lateral rays are 

 reduced to insignificant dimensions — the two together not occupying half the 

 space of either of the others. This is not a mere individual abnormality, but 

 is constant in four specimens, and is correlated with other characters distinc- 

 tive of the genus. Unequal rays are found in other groups, especially among 

 the Inadunata including their recent representatives, where the anterior ray is 



