H ETERO CRI N I D AE 
85 
Suborder FISTULATA Wachsmuth and Springer 
Monocyclic or dicyclic. Posterior interambulacrum usually more or less 
extended into a strongly plated anal tube or sac. Arms pinnulate or non- 
pinnulate, usually uniserial, but biserial in some of the later genera. Tegmen 
composed of numerous plates, either orals with supra tegminal ambulacra pass- 
ing over their edges, and interambulacra, or more or less undififerentiated plates. 
Family HETEROCRINIDAE Zittel 
Monocyclic. Basals five. One or more of radials compound. Anal x 
usually resting on left shoulder of r. post. R, supporting a tube. Arms five, 
non-pinnulate, uniserial, dichotomous or heterotomous, or may bifurcate with 
ramules; proximal IBr usually full width of RR. Tegmen little known. 
MYELODACTYLUS Hall 
Plate 2^ 
Myelodactylus Hall, Pal. New York, 2, 1852, p. 191. — Anselin, Icon. Crin. Suecc., 1878, p. ii. — Wachsmuth 
and Springer, Rev. Pal., i, 1879, p. 146. — Weller, Bull. Chicago Acad. Sci., 4, 1900, pp. 45, 68. — 
Springer, Unusual forms of Fossil Crinoids, Proc. U. S. Nat. Mus., vol. 67, 1926, pp. 3-32, pis. 1-6. 
Herpetocrinus Salter, Catal. Camb. Sil. Foss., 1873, p. 118. — Bather, Crin. Goth, 1893, p. 36, te.xt-fig. 12, 
pi. I, figs. 24-77; Amer. Geol., 1895, p. 214; Nat. Sci., 12, i8g8. p. 339; Treatise on Zocl., 3, 1900, p. 146, 
fig. 49. — Zittel-Eastman, Textb. Pal., 2d ed., 1913, p. 212. — Bassler, Eibliogr. Index, 1915, p. 844. — 
Thomas, Proc. Iowa Acad. Sci., 26, 1919, p. 481. 
An Inadunate crinoid with coiled stem, more or less completely enveloping 
the crown, circular in proximal region; bilaterally symmetric, elliptic or sub- 
crescentic in middle and distal regions ; with two rows of cirri along the margins 
or back of the bilateral part. Crown similar to that of locrinus. 
Genotype. Myelodactylus convolutus Hall. 
Distribution. Silurian to Lower Devonian ; America, England, Gotland. 
Having recently in the paper on “ Unusual Forms of Fossil Crinoids,” above cited, dis- 
cussed this remarkable form and its congeners at considerable length, with descriptions and 
illustrations of the known American species, it is unnecessary to go into details concerning it 
here. Inasmuch, however, as some of the most important of the new material belongs to the 
Tennessee fauna under consideration, and for completeness must be given a place in this 
work, it has been thought best to reproduce a selection from the principal figures, assembled 
upon a single plate the better to show at a glance the variety and range of this greatly special- 
ized genus. To this end a list of the species as figured will be given with a brief resume of the 
salient characters, while for more complete information the reader is referred to the paper 
alluded to. 
