MELOCRINID &. 327 
Barris apparently regarded the two forms as representing the same species. 
However, on examining a large number of specimens of both types, we find 
a separation necessary. In S. Barrisi the basal disk is small, extending but 
slightly beyond the column, and the facet for the reception of the column is 
surrounded by a heavy, circular rim. In S. triangulatus the disk is quite 
large, the top stem joint occupying less than half its diameter, and it has no 
rim around the facet. The interambulacral spaces of S. Barrisi are much 
more depressed, the ambulacra more abruptly protruding, the orals larger, 
and the interambulacral plates flat and less numerous. 
HADROCRINUS Lyovy. 
1869. Lyon; Trans. Amer. Philos. Soc., Vol. XIII, p. 445. 
1881. W. and Sp.; Proceed. Acad. Nat. Sci. Phila., p. 396 (Revision, Part IT., p. 222). 
1889. S. A. Miter; N. A. Geol. and Paleont., p. 252. 
Calyx very large; the dorsal cup saucer-shaped, its lower portions con- 
eave. Basals small, completely covered by the column and placed at the 
bottom of the cavity; they are closely anchylosed, and their suture lines are 
visible only upon the inner floor. Radials pentangular, the lower half bent 
inward, the upper stretched outward. Costals one, pentagonal. Distichals 
and palmars are always represented in the calyx, and sometimes post-palmars ; 
each order by two to four successive plates of the same size as the costals; 
the uppermost plate excavated to form a facet, which is directed laterally, 
and supports the arms. Interbrachials variable in number and irregular in 
their arrangement, as large as the adjoining brachials. The anal. side may, 
or may not have an additional plate in the second row. Interdistichals one 
to three, longitudinally arranged, as large as the interbrachials. Structure 
of ventral disk, arms and anus unknown. Column very large near the calyx 
(Lyon), round, tapering rapidly, and giving off cirri; axial canal circular at 
the top of the column, but pentalobate farther down. 
Distribution. — Only known from the Upper Helderberg group. 
Type of the genus. — Hadrocrinus discus Lyon. 
Rtemarks.— The specimens are all imperfect, and no satisfactory definition 
of the genus can be given. It is allied to Stereocrinus in having but one 
costal to the ray, but differs essentially in the number and arrangement of 
the interbrachials, and in having sometimes an anal plate. Lyon’s descrip- 
tion of the column needs confirmation, being made from detached pieces. 
His 7. pentagonus is defined from a single ring of plates, and the species 
cannot be recognized, 
