17G PALAEONTOLOGY OF ILLINOIS. 



radials. The arms are usually strong and variously branched, 

 each division being probably always composed of a single range 

 of pieces. 



The vault is composed of a comparatively small number of 

 pieces, and is provided with a central oral, and an entirely 

 distinct, lateral anal opening, one of which is, according to 

 de Koninck and Lehon's description, and Austin's figures, pro- 

 duced in the form of a short proboscis. 



In the number and arrangement of the pieces composing that portion of the 

 body below the arm bases, this genus is very similar to Poteriocrinus, but in 

 the structure of the upper part of the body, these two groups are remarkably 

 distinct. For instance, the vault of Cyathocrinus is comparatively very low, 

 or merely arched a little above the attachment of the arm bases, and provided 

 with a central or subcentral oral opening, sometimes a little produced as a short 

 proboscis, and an entirely distinct lateral anal opening ; while in Poteriocrinus 

 the whole summit is enormously extended in the form of a great proboscis, 

 often as wide as the entire body, or sometimes much wider above, and nearly or 

 quite as long as the arms, with (according to Austin) a single opening at its 

 summit. These we regard as the most important differences between these two 

 genera. 



Other less striking differences, however, are usually observable between the 

 species of these groups, .such, for instance, as the more obconical form of the 

 body in the typical species of Poteriocrinus, which also generally have one or 

 two more anal pieces included in the walls of the body, below the summit of 

 the first radial pieces, than in Cyathocrinus. Again, in most cases, they like- 

 wise have one of the first radials (that on the right of the anal series) resting 

 upon the truncated upper extremity of one of the subradials, instead of all 

 alternating with the subradials as is usually the case in Cyathocrinus. These 

 latter peculiarities of structure, however, are not constant in either of these 

 groups. 



The genus Cyathocrinus, as properly restricted, seems to be, as maintained 

 by M. M. de Koninck and Lehon, Pictet and some others, mainly, if not entirely, 

 confined to the Carboniferous and Permian rocks. We are aware several species 

 have been referred to it from the Silurian and the Devonian strata, but nearly 

 all of these not clearly belonging to other established genera, present more or 

 less marked differences in the parts known, from the typical Carboniferous 

 species; while none of them are known to possess the two distinct openings in 

 the summit, characterizing the typical species of Chathocrinus. 



The only Silurian species known to us, referred with probable correctness to 



