124 
SMITHSONIAN INSTITUTION 
scribed without figure from an isolated calyx only, without any distinctive char- 
acters. As there is nothing- in the description to establish priority, it is deemed 
proper tO' hold it for the species which is recog'iiizable. 
This is the dominant species of the genus in the Burlington limestone, where it is found 
in excellent preservation. It is represented in the collection by upwards of 20 specimens, which 
exhibit some of the variations to be expected in a prolific species. Normally it has 4 axil-arms, 
but there may be 3 or 5 ; the median arm branches on the 6th to the 8th or 9th brachial, usually 
with a single bifurcation, but sometimes with two ; the ramules of the lateral arms usually 
occur at intervals of 4 brachials, but occasionally 3 or 5, mostly in two series but may be three, 
always exposed. The form of the cup changes with maturity of the individual, that of the 
younger ones being higher and narrower than in the older ; as a rule the cup is more elongate 
than in other species, the average of four specimens being 9 mm. height by 8.5 mm. width at 
the hinge. A striking example of the difference in this and other respects due to age is fur- 
nished by an extremely young individual associated with this species and presumably be- 
longing to it, which I have illustrated on plate 30. The complete crown is only 12 mm. high, 
and the stem, also about complete, 22 mm. long ; the cup is 4.5 mm. high, 2.5 mm. wide across 
the 1. ant. side at the top, diminishing to 1.7 mm. at the hinge ; on the 1. post, side, as originally 
exposed, there is but one arm, unbranched, and on the anterior side, as seen by removing 
the crown from the matrix, there are two branches, l^esides a curving anal tube, also visible 
from the first position. Enlarged figures were made from both these views (pi. 30. figs. 3, 3a). 
Horizon and locality. Tapper Burlington limestone, Mississippian ; Burlington. Iowa. 
Halysiocrinus nodosus (Hall) 
Plate so, figs. 14-21 
Chcirocrinus nodosus Hall, 13th Rep. New York St. Cab. Nat. Hist., i860, p. 124; Phot. Plates of Cri- 
noids, 1872, pi. 6, fig. 15 . — Halysiocrinus nodosus Ulrich, 14th Ann. Rep. Geol. Surv. Minnesota, 
1886, p. 1 12 . — Deltacrinus nodosus, Spr., Smithsonian Misc. Coll., 76, 1923, pi. 5, fig. 21. 
This species may be taken to represent the culmination of the family in 
the Lower Carboniferous. At least one species from a later horizon is known, 
which is rare and shows a reversionary tendency. The present form is selected 
for illustration to complete the picture of the type we have been discussing, 
because it is the most striking species of all the Carboniferous in its characters, 
is found in remarkably fine preservation, and in such abundance as to offer 
excellent facilities for the study of variations within the limits of a well defined 
species. The large size, robust habitus, sharp nodosity, curvature and frequent 
branching of the median arm, and free exposure of main axil-arms, are con- 
spicuous and decisive characters. 
I am illustrating it on plate 30 rather fully, first, because the recumbent habitus, with 
the crown pressing closely down over the stem, is so admirably shown, for example by figure 19 
in which the arms seem actually to envelope the stem in a presumably horizontal position. In 
figure 17, the suggestion that the crown might close down like a barnacle is brought forcibly 
to mind ; it is here strongly curved in the plane of its bilateral symmetry, with the large median 
arm at the perimeter ; it is a peculiar posture, which might be described by the word “ humped,” 
and the resulting general form, when seen from the side, is semi-elliptical. This is the typical 
form for the species, and while there are some specimens not otherwise distinguishable in 
which the “ hump ” is not so pronounced, it is a fact that out of about forty well preserved 
