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 Lyok. 



1869. Lyon; Trans. Amer. Philos. Soc, Vol. XIII., p. 445. 



1881. W. and Sp. ; Proceed. Acad. Nat. Sci. Pliila., p. 396 (Revision, Part II., p. 222). 



1889. S. A. Miller; N. A. Geol. and Paleeont., p. 253. 



Calyx very large ; the dorsal cup saucer-shaped, its lower portions con- 

 cave. 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. Eadials 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-palm ars ; 

 each order by two to four successive plates of the same size as the costals ; 

 the uppermost plate excavated to form a facet, w^hich 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 ojEf 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. 



Bemarks. — 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 K pentago7ius is defined from a single ring of plates, and the species 

 cannot be recoofnized. 



