MELOCRINID^. 325 



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 Barkis. 

 Plate XXV. Figs. 8a, h. 



1878. Barris; Proceed. Davenport Acad. Nat. Sciences, Vol. II., p. 261, Plate 11, Figs. 1, 3. 

 1881. W. and Sp. ; Revision Palgeocr., Part II., 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 w^alls 

 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 triadactyliis, 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 ^y^ 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. Eadials 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- 

 4^nown. 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, 



