1879.] NATURAL SCIENCES OF PHILADELPHIA. 307 



the pinnulae and in the structure of the ambulacra! groove, which, 

 instead of those appendages, is provided with two rows of from 

 two to five successive movable plates, alternately arranged on 

 opposite sides, by means of which the furrow could be opened to 

 the surrounding element, or shut off from it. 



Anal plate one, resting upon the superior edge of the truncated 

 basal and between two radials. None of the succeeding plates in 

 the series are embraced in the calyx ; they are much smaller and 

 form a part of the ventral sac. 



The vault is composed of five large oral plates, joining later- 

 ally, which fit in between the inflected margins of the two radial 

 plates, leaving in the oral centre an open space, which in perfect 

 specimens is completely covered by the apical dome plates. The 

 food groove and ambulacral canal are also arched over solidly by 

 two rows of alternate plates which connect with the movable 

 covering of the arm furrow. The ventral sac extends to about 

 one-half the height of the arms, and is composed of a large num- 

 ber of small plates not so regularly arranged as in Poteriocrinus. 

 Its communication with the main body internally is maintained 

 by means of a passage pierced through the posterior oral plate. 

 The sac is stronger in the Silurian species, and in them only have 

 pores and slits been observed. 



Column round, with alternate larger and smaller joints in its 

 upper portion; central passage scarcely of medium size. 



Geological position, etc The genus existed in the Lower Silu- 

 rian, was well represented in the Upper Silurian, and attained its 

 greatest abundance in the Subcarboniferous. Beyond the age of 

 the Keokuk limestone no trace of it has been discovered, and to- 

 ward the close of that epoch the general aspect of the species un- 

 derwent a marked change, the later species being large, and the 

 arms, which before were delicate, became very robust at the bases. 

 (See Wachsm. & Spr., Proc. Acad. Nat. Sci. Phila., 1878, p. 257.) 

 According to Murchison and others, Cyathocrinus is represented 

 in the Permian, but nothing has been discovered upon which to 

 base this opinion beyond the fragments of columns, which afford 

 no reliable proof. So far as ascertained there are 4 species from 

 the Lower, 19 from the Upper Silurian; a single one from the 

 Devonian; 28 from the Subcarboniferous, making in all 53 species, 

 of which 27 have been found in Europe, and 26 in America. 



