1890.] NATURAL SCIENCES OF PHILADELPHIA. 351 



that of »a Platyervius. He is inclined to believe (Chall. Rep. on 

 Stalk. Crin., p. 180), "that the vault of a Platycrinoid corresponds 

 collectively to the orals, interradials, ambulacral and anambulacral 

 plates of Neocrinoids." He regards (p. 178) the peripheral portion 

 of the " vault," by which he means the zone between the so-called 

 summit plates and the radials, as generally corresponding to the large 

 interradial of Cyathocrinus, and to the single interradial of Coecocri- 

 mis. As to the plate of Cyathocrinus we cannot agree with him for 

 reasons which will be stated further on. That of Coccocrinus 

 obviously represents the " calyx interradials " of Platycrinus, and 

 not a plate of the peripheral zone. 



In referring to MarsujAocrinus he says (p. 176), "I have a very 

 strong impression that the so-called vault of this genus is really the 

 strongly plated ventral perisonie," and " I cannot see any such essen- 

 tial difference between it and the plated disk of Pentacrinus wyville- 

 thomsoni or of many Antedons (PI. XVII, fig. 6 ; PI. LV.) as would 

 lead to the supposition that the homologue of the latter is to be 

 sought for beneath the vault of 3Iarsiipiocrinus." He then alludes 

 to the closure of the n)outh, and to the covering pieces proceeding 

 from the perisome, which may have been immovably closed down 

 over the food grooves : " They were thus converted into tunnels, but 

 were still ' external,' in the sense of not being covered by a ' tegmen,' 

 as those were which formed the tubular skeleton beneath the vault 

 of the Actinocrinidae." 



We have pointed out before that the covering pieces are exposed 

 not only in the Platycrinidae, but quite frequently also in other fami- 

 lies of the Camerata, exceptionally even in the genus Actinocrinus. 

 Now, if it were true that in Crinoids in which the calyx ambulacra 

 are entirely subtegminal, and not only the ambulacra but the whole 

 disk is covered in by a structure " sui generis" and on the contrary, 

 in forms in which the ambulacra are exposed there is no vault, and 

 the plates in which the ambulacra rest form the disk, it seems to us 

 that the two groups should be distinguished as separate orders ; and 

 it would seem to follow that all attempts heretofore made toward 

 classifying the Crinoids would be altogether arbitrary and worthless. 

 It was these considerations which led us to believe that the integu- 

 ments in both cases must be the same thing, either a vault or a disk, 

 the plates either all vault pieces or all perisomijc. The evidence 

 seemed to be conclusive that the disk, at least in some groups, was 



