96 
SMITHSONIAN INSTITUTION 
Now these progressive modifications that have occurred in the phylogeny of the familv 
are paralleled and recapitulated to some extent in the life history of the individual, as will be 
seen in the description of some of the later species. While not absolutely uniform, but marked 
by some exceptions and recessions, yet in general the facts indicate a well defined progression 
within this family from the early to the later geological stages comparable to that which 
occurred from the young to the mature stages of individual growth in the later forms. 
The Anal Tube 
The anal tube, although often concealed from view by the posterior arm branches, is 
usually large, and is composed on the posterior side of massive plates, somewhat resembling 
brachials ; they do not entirely enclose the tube, but are crescentic in section, outwardly curved 
and leaving a space at the anterior side which is filled by an integument of small plates, similar 
to what has been described in Catillocrinns and Synbathocrinus.^ The anal .v is always a large, 
convex plate, constructed like the tube plates. The statement of Wachsmuth and Springer^ 
that the tube extends to the tips of the arms and perhaps be3'ond their limits, was doubted 
by Bather (Crin. Goth, p. 71), but is confirmed for the Silurian species by the new material 
(pi. 29, figs, yb, 8 b, JSb). Of this the most remarkable example of all is the new species 
Cremacrinus tubuUfcrus, in which the tube is not only of great size, but may bend outward 
like a bow, so that for a part of its length it is free from the arms.® 
Shifting of the Stem 
Aside from the reversal in direction, the shifting of the stem in this family is a remarkable 
pbenomenon, which I think has not been noticed heretofore by authors. Correlated as it is 
with other changes of first rank, it becomes a character of much significance in the evolution 
of the group. It has to do directly with the equilibrium of the organism, which is so notable 
a feature among paleozoic echinoderms. 
In the 5-rayed ancestral form, the plane of bilateral symmetry is from the anal tube to 
the anterior ray, leaving two arm-bearing radials on each side of it. When in connection 
with the bending of the crown upon the base the right posterior arm is eliminated, and the 
reversed stem falls into its place, the left anterior arm becomes the dominant factor on the 
anterior side, and the plane fixed by these two bisects tbe calyx so as to leave an arm and the 
anal tube on one side to balance the two arms remaining on the other. With the elimination 
of one of these two arms in the next stage, this balance would be disturbed, but for the fact 
that it is at the same time restored by the sternward torsion of the parts from the right to fill 
the vacancy, and the consequent shifting of the stem to a median position directly in front 
of the anal tube, thus establishing a plane of perfect bilateral symmetry with one large arm- 
bearing radial on either side. This being the condition of stable equilibrium, it is maintained 
to the end of the group. Thus it is seen that the location of the stem in its two positions has 
a definite connection with the changes accompanying the progressive modifications within 
the family, and marking the broad separation of Form A from all other forms. 
The Hinge 
The outstanding character of this family, by which it is broadly set off from all other 
known crinoids, is the mobility of the crown upon the base incident to the connection of the 
radials with the consolidated basals by muscular articulation instead of the usual .suture, by 
1 Springer, On the Family Catillocrinidae, Smithsonian Misc. Coll., Vol. 76, No. 3, 1923, pp. 17-20. 
2 Revision of the Palaecrinoidea, pt. 3, 1886, p. 281. 
2 See also Miss Goldring’s description of the tube in Halysiocrinus secundus, Devonian Crinoids of 
New York, 1924, p. 339, pi. 41, fig. 6. 
