244 PROCEEDINGS OP THE ACADEMY OP [1885. 



Cyathocrinus multiradiatus from Crawfordsville, of which the 

 one is figured (PI. 4, fig. 6), the entire surface of the interradials, 

 and also the circum-oral space, is covered by minute plates, 

 except at one end (see figure) where the plate underneath is 

 exposed -to view. The structure is similar in Cyathocrinus 

 iowensis from the Lower Burlington limestone (PI. 5, fig. 7), hut 

 there the plates closing the peristome consist of eight consider- 

 ably larger pieces, placed around a central one, arranged in pairs, 

 of which each pair corresponds in form and position to one of 

 the four large proximals in other genera. 



In the above specimens, the so-called orals are covered along 

 their sutures by well-defined ambulacra, lined by side-pieces 

 and covering plates, and these connect laterally with the small 

 tegminal plates which we have described. That all surface plates 

 in these species are perisomic, nobody will doubt after consulting 

 our figures, and that the plates supporting them are interradials 

 and not orals, is proved by the fact that the} T surround the peris- 

 tome, but do not cover it, and are succeeded by numerous other 

 plates. 



This, however, was not the structure of the Inadunata gener- 

 ally, or even of all Cyathocrinidae. Angelin figures from the 

 Silurian of Sweden (Icon. Crin. Suec, PI. 23, figs. 10 6, 11), two 

 specimens under the name of Cyathocrinus alutaceus, in which 

 the interradials (orals) were exposed, and not covered by plates. 

 The} T have a central piece, surrounded by four large proximals, 

 and there are, alternating with them, live conspicuous radial dome 

 plates, with numerous irregular pieces along the posterior or anal 

 side, which join the central plate, and extend outwards, forming 

 a short protuberance, composed of small pieces. There are at 

 the surface no traces of ambulacra, and the whole structure ven- 

 trally is almost identical with that of certain forms of Platycrinus; 

 while the dorsal side of the species shows clearly the characters 

 not only of the Cyathocrinidae generally, but the detail structure 

 of the genus Cyathocrinus. The total absence of ambulacra upon 

 the surface proves that in this species the disk was subtegminal. 

 covered by the plates which have been heretofore called orals, but 

 which are identical with the first interradial plates of Platycriiuis, 

 and with the lirsl interradials of Actinocrinus and other Camarata. 

 The structural identity with all these plates proves that the inter- 

 radials of the Cyathocrinidae, and the deltoids of the Blastoidea, 



