348 PALEONTOLOGY OF ILLINOIS. 



depressed vault, a broad, more or Jess distinctly ten-angled disc, from 

 the margins of which the numerous long, slender arms arise, without 

 bifurcating after becoming free.t Indeed, with rare exceptions, the rays 

 can scarcely be said to bifurcate, properly, after the division on the third 

 primary radials, though each main division continues on out, throwing 

 off alternately on each side brachial pieces in close contact with each 

 other, until, at last, it terminates in a single free arm. Each of the arms 

 commences abruptly as a double series of small alternating pieces im- 

 mediately on the last fixed brachial piece, without an intermediate, 

 series of free single pieces extending entirely across. Some of the 

 species, such as S.periimbrosus, have but a very small simple opening 

 situated subcentrally, or more or less excentrically towards the anal side, 

 and penetrating the flattened vault obliquely, so as to be directed for- 

 ward or away from the anal side ; while others, like 8. liratus, have a 

 long erect, subcentral tube, or so-called proboscis, sometimes recurved 

 at the end. The column is known, at least in the species provided with 

 proboscis, to be peculiar in being composed of very thin segments, a 

 part of which, at regular intervals, project out beyond the others, and 

 send up and down, at equal distances all around, five external, thickened 

 processes or ribs, apparently as a natural provision to give it strength, 

 without destroying its flexibility. 



Then we have the Act. ventricosus group, which not only agrees with 

 the S. perumbrosus section of Strotocrinus in having merely a very small 

 subcentral or exceutric opening in the vault, without any traces of a 

 proboscis, but also, to a considerable extent, in the manner in which the 

 subdivisions of the rays are given off 5 but differs in having these sub- 

 divisions not in contact so as to form a disc, but divided by narrow 

 interradial. anal, axillary, and sometimes interbrachial sinuses, the 

 former of which often extend quite into the body. The species of this 

 group also differ from the typical forms of Stroiocrinus in having the 

 body shorter below the arms, and the vault generally more ventricose, 

 and provided with external furrows radiating from the middle to the 

 anal and interradial, sinuses. So far as yet known, the species of this 

 type have rather stouter and less numerous arms than we see in Stroto- 

 crinus proper, but generally more than we see in Actinocrinites. In both 

 groups of Strotocrinus the arms are, as in Actinocrinites, provided with 

 numerous pinnulse, or so-called tentacles, but here they seem to be 

 always armed with minute spiues directed more or less obliquely upward 

 from the upper margins. 



From Actinocrinites the A. ventricosus group not only differs in being 

 without any traces of a proboscis, but in having its ventricose, furrowed 

 vault composed of numerous minute pieces ; and the divisions of its 



t Some of the species have as many as seventy to eighty arms. 



