MELOCRINID. 825 
Arm openings, so far as observed, two to the ray; arms biserial. Interradial 
spaces wide, containing three ranges of plates in the dorsal cup; the first 
and second range composed of one plate each; the third of two or three; 
followed by large cuneate plates in the disk. Ventral disk rather flat, the 
interambulacral spaces depressed, containing one or more slit-like openings 
at each side of the arms. Ambulacra subtegminal, but their course indicated 
by elevations upon the disk. Orals large and arranged as in Dolatocrinus. 
Anus subcentral, apparently at the upper end of a tube. Column round; 
central canal large and pentalobate. 
Distribution. — This genus has been observed only in Michigan and Iowa. 
The typical species came from the Hamilton group; but we have in our 
collection a specimen apparently of another species, from rocks considered 
to be Upper Helderberg, of Waterloo, Iowa; it is not, however, sufficiently 
perfect for description. 
Stereocrinus triangulatus Barris. 
Plate XXV. Figs. Sa, b. 
1878. Barris; Proceed. Davenport Acad. Nat. Sciences, Vol. II,, p. 261, Plate 11, Figs. 1, 2. 
188]. W. and Sp.; Revision Paleocr., Part I., p. 127. 
A rather large species. Dorsal cup shallow-basin shaped, twice as wide 
as high; the truncate lower part formed by the basals, radials, the larger 
half of the costals, and a large part of the first interbrachials; the lateral walls 
of opposite sides nearly parallel; the extreme upper end slightly constricted, 
and the interradial spaces a little depressed between the arm bases. Orna- 
mentation resembling that of Dolatocrinus triadactylus, the surface being 
covered by similar sets of parallel ridges, but less prominent. Only one of 
each set passes from centre to centre of the plates, the others being inter- 
rupted. By means of these ridges the whole surface of the dorsal cup is 
divided up into numerous triangles, each one enclosing one or two smaller 
ones. There are five such sets of triangles around the basals, three to each 
set, and similar triangles are formed in the upper part of the calyx. 
Basals largely projecting beyond the column; central canal large, penta- 
lobate. Radials a little larger than the costals, all wider than long. Dis- 
tichals 2 X 10; the first rather large; the second short, lunate. Arm bases 
projecting, arranged in pairs. Arms two to the ray; their structure un- 
known. First interbrachial a little larger than the radials; the second about 
half the size of the first, followed by two or three plates in the third row, 
