1885.] NATURAL SCIENCES OF PHILADELPHIA. 247 



or beyond the succeeding ring of interradials. In either case, 

 however, the covering-plates join laterally with the interradials, 

 and accommodate themselves, more or less, in form and size, to 

 the surrounding plates, so much, indeed, that frequently they 

 attain the same rigid form as the true vault pieces (PI. 7, figs. 5, 

 7, 8). Sometimes, however, as in the case of Marsupiocrinus 

 ccelatus, the alternating plates retain their original form and deli- 

 cate structure, while in the same genus, in Marsupiocrinus 

 Tennessese (PI. 8, fig. 7), they are as rigid as the interradials. 



For pi-oof that our descriptions of the alternating plates, and 

 the ambulacral tubes generally, are based upon actual observa- 

 tion, we refer to the casts of Dorycrinus (?) (PL 4, fig. 5), Stroto- 

 crinus (PI. 4, fig. 4), and Platycrinus (PI. 5, fig. '.)), in all of which 

 the ambulacra, at some distance before entering the peristome, 

 are covered up in the cast and are visible upon the surface only 

 at or near the arm bases. The cast of Platycrinus, which we have 

 illustrated, shows beautifull}- the alternate arrangement of the 

 covering plates, which pass out from beneath a belt of large 

 interradials. Looking at this figure we do not see how Carpenter 

 can any longer maintain that Platycrinus possessed no tubular 

 skeleton, and that the upper interradials are anambulacral plates. 

 The specimen will also convince him that there are in this genus 

 upon the surface of the cast no " elevated rounded ridges, almost 

 like strings overlying the surface," as he imagined (Chall. Rep., p. 

 179), and which, he thought, represented " the open food grooves 

 of recent Crinoids." Among the twelve or more casts of Platy- 

 crinus which we examined from Mr. Rowle}'^ collection, not one 

 bears that string-like structure, and in all of them the ambulacral 

 tubes are placed around the peristome at a distance from the 

 vault. That even in the Actinocrinidas those strings which we 

 noticed upon the casts do not represent organs connected with 

 the food grooves, will be shown elsewhere. 



Among Actinocrinidae, and probabby in other families, the 

 covering plates sometimes penetrate the interradials in a similar 

 manner as in the Platycrinidae, and this is so even in the genus 

 Actinocrinus. Actinocrinus stellaris, from the Mountain lime- 

 stone of Belgium, has a row of alternating plates covering the 

 food grooves, a character not well shown in Be Koninck's figures • 

 although the arrangement of the plates is very regular in the 

 specimens, and almost identical with that of certain species of 



