358 PROCEEDINGS OF THE ACADEMY OF [1890. 



4-5 ; but while in all of the former species the ambulacra rest between 

 small irregular perisomic plates Avhich ])ass up from the upper ends 

 of the radials, Euspirocrinus has four large " interradial " plates, 

 and the corresponding space of the posterior side is taken up com- 

 pletely by the ventral tube. Whether these four plates were covered 

 like those of C. iowensis and C. gilesi cannot be ascertained from the 

 specimen, nor can we say whether the Silurian Cyathocrinidae 

 generally had a large plate beneath their smaller ones ; but we are 

 convinced that such a plate is present in all Subcarboniferous species 

 of CyatJiocrinus. 



Conceding now that those plates of Cyathocrinus are not orals, what 

 are they ? Interradial plates ? It seems to us the fact that they support 

 the ambulacra and are covered by perisome, proves as completely that 

 they are not interradials as that they are not orals. If they were 

 calyx interradials, the " vault" would be placed beneath the disk, 

 Avliile if they were perisomic plates there would be two disks on top 

 of one another. Besides, the plates are subtegminal and decidedly 

 subambulacral, and the question arises are they not wholly or in 

 part subambulacral plates. 



"We have examined the two specimens of Cijathocrinus iowensis 

 figured by Meek and Worthen (Geol. Rep. Illinois, Vol. V, PL 9, 

 figs. 13 and 14), which are now in the Museum of Comparative 

 Zoology at Cambridge. After comparing with them our specimens 

 from Indiana, and several from Burlington, we became convinced 

 that the consolidating plates, as we had called them, consist not of 

 five but of seven plates, one to each of the four regular interradii, 

 and three to the posterior one. Of the three latter plates, the mid- 

 dle one is larger, and occupies the space between the ambulacra and 

 the ventral tube, the other two being altogether subambulacral. 

 The middle plate, which we find in most excellent preservation in 

 the Indiana specimens, is not covered by other plates, but is through- 

 out profusely perforated, in a somewhat similar manner as the mad- 

 rei)orite of other Echinoderms, and in our opinion, must have per- 

 formed similar functions. 



Considering now that in Cyathocrinus iowensis, and in the other 

 Subcarboniferous Cyathocrinidae in which the " madreporite" is 

 represented, the tube is imperforate, and that, on the contrary, in 

 Euspirocrinus spiralis the whole space which in other forms is oc- 

 cupied by the madreporite, is taken up by the ventral tube, and that 

 this is profusely perforated, it seems natural that the single plate of 

 the former should have performed the functions which in the latter 



