MELOCRINID^. 313 



arm bases slightly projecting and lobed, giving to the cross-section an 

 obtusely pentagonal outline. Ventral disk low-pyramidal^ the interambu- 

 lacral spaces flat or slightly depressed, and the plates apparently without 

 ornamentation. In the dorsal cup the plates are traversed by a large num- 

 ber of very fine radiating ridges^ passing from plate to plate, four to five 

 from each side, which are well defined near the outer margins of the plates, 

 but become more indistinct along the median portions. Interbrachial plates 

 a little convex, the plates of the rays from the margin of the basal concavity 

 to the middle of the second costals surmounted by keel-like, very conspicu- 

 ous protuberances, which rise from the lateral margins of the plates, at first 

 very gradually, but near the middle of the plates almost perpendicularly, 

 forming knife-like edges at the outer end. The ridges are continued upon 

 the distichals, but above the costals lose their knife-like character. 



Basal disk small ; restricted to the lower part of the concavity, which is 

 completely filled by the upper end of the column. Eadials a little larger 

 than the costals, their lateral sloping upper faces considerably shorter than 

 the corresponding lower. Distichals two — sometimes three — the upper 

 one short and lunate, supporting the primary arms, two to the ray. First 

 interbrachial larger than any of the other plates, nearly as wide as long; 

 followed by a good sized single plate, and this by two ranges of three plates 

 each, which rest between the arm bases. In the tegmen there are two large 

 cuneiform plates to each interambulacral space, which are faced by two or 

 more smaller pieces with four slits. Two similar plates and two slits overlie 

 the interdistichal s^oaces. Orals large, pushed anteriorly. Radial dome 

 plates quite conspicuous ; three of them resting within the re-entering angles 

 of the orals, the two others against the large anal tube. 



Horizon and Locality. — Upper Helderberg group ; Falls of the Ohio, near 

 Louisville, Ky. 



Type in the Lyon collection, Jeffersonville, Ind. 



Remarhs. — Lyon described the basals as "horizontally dispo,sed," and 

 " not concealed in the pit." We conclude from his type specimen that he 

 mistook the extended outer edges of the top stem joint for the margin of the 

 basal disk, and its nucleus for the column. Another specimen in the Lyon 

 collection shows the inner floor of the basal disk, which appears to be 

 tripartite. 



40 



