1881.] NATURAL SCIENCES OF PHILADELPHIA. 385 



Underbasals five, small, spreading horizontally, and hidden 

 by the column, though sometimes slightly visible beyond its 

 periphery, Basals five, equal, comparatively large, heptagonal, 

 upper side truncate. Primary radials 3X4, generally as high 

 as wide, decreasing in size upward, the series separated laterally 

 by interradial plates ; the first heptagonal ; the second from 

 quadrangular to hexagonal, according to the size of the first 

 radials ; the third irregular in form. The latter, in the more 

 depressed Subcarboniferous forms, supports only from one to two 

 secondary radials ; in Devonian species, there are sometimes three. 

 None of the plates project outward, the upper one, which is also 

 the arm-bearing plate, is excavated at its upper edge, and this, 

 together with a similar excavation in the corresponding dome 

 plates, forms an oblong ambulacral passage, two to each ray, 

 which are separated b}- an interbrachial plate. The arms of the 

 difierent rays are widely separated ; they are long, delicate, 

 cylindrical, branching, and constructed of two rows of alternate 

 plates, which interlock from the base up. Pinnules small. 



Interradial and anal areae very wide and closely similar, the 

 latter having sometimes one or two additional plates without dis- 

 turbing the general symmetry of the body. The first plates, 

 which are large, rest upon the truncate upper side of the basals, 

 and laterally between the first radials ; the second and third series 

 consists of from two to three plates each, and the plates are of 

 comparatively large size, those of the succeeding series much 

 smaller, and passing gradually into vault pieces. 



Vault flat, compressed and narrow, owing to the constriction at 

 the upper part of the calyx ; composed of a large number of 

 irregular pieces, among which the apical dome plates are not 

 easily distinguished ; radial portions generally protuberant. Anus 

 excentric, protruding like a proboscis, and consisting of almost 

 microscopic plates, possibly capable of expansion or contraction 

 by the animal. 



Column round, and composed near the body of very uneven 

 plates ; perforation small, pentagonal. 



Geological Position^ etc. — The genus Rhodocrinus ranges from 

 the Devonian to the middle portion of the Subcarboniferous, both 

 in America and Europe. 



We recognize the following species : — 



26 



