27 



Billings described the genus Retiocrinus as follows: "This re- 

 markable genus has no perfectly formed plates. The cup consists 

 of a reticulated skeleton, composed of rudimentary plates, each 

 consisting of a central nucleus, from which radiate from three to 

 five stout processes. Of such plates there are five in the basal 

 series, five in the subradiaL and five in the radial series. On the 

 azygous side the subradial has five processes; the others have four 

 each." 



The general appearance of the genus is that of a reticulated 

 skeleton, as Billings described it, but the plates are all perfectly 

 formed and not by any means rudimentary. The interradial areas 

 are abruptly sunken, and, as shown in our specimen, small plates 

 connect the inner sides of the radials, with a central body, that is 

 also covered with very small plates, which, if not so deeply de- 

 pressed, would look somewhat like the interradial areas in 

 Gaurocrinus, and if they were not depressed, at all, they 

 would look like the interradial areas of Glypiocrinus, except they 

 are covered with smaller plates. The ambulacral furrows in 

 Glypiocrinus and Gaurocrinus enter the top of the calyx and 

 unite centrally immediately below the vault, while in this genus 

 they appear to follow down the radial series and to unite, at the 

 lower part of the calyx, or between the subradials. The structure 

 indicates this, but the channels forming the union have not been 

 discovered. We have called this central cavity a balloon-shaped 

 body, only for the purposes of description, for it is the same as 

 the cavity of the calyx in other genera. Its form is only more 

 balloon-shaped than an ordinary calyx because it is small in the 

 lower half of the calyx. Even if the ambulacral furrows unite at 

 the summit of the calyx, and not below, as we have thought most 

 probable, the other structural parts are sufficient to warrant the 

 generic name. No comparison of this species with any other is 

 necessary to distinguish it, but it is interesting to note the find- 

 ing of a new species in this rare genus, several hundred miles 

 distant from the typical locality. 



Found in the Trenton Group, in Mercer County, Kentucky, and 

 now in the collection of Wm. F. E. Gurley. 



