1885.] NATURAL SCIENCES OF PHILADELPHIA. 279 



central plate has a somewhat elongate form, and which he 

 described as having five orals. The summit plates in both 

 genera are subtegminal, being covered completely by interra- 

 dials, and the same was probably the case in the allied Ichtbyo- 

 crinidse, at least in their earlier forms. Reteocrinus and Xeno- 

 crinus were evidently in a similar condition, but it is not known 

 whether they had summit plates beneath the interradials or not. 

 Ghjptocrinus and most of the Silurian genera of the Camarata 

 had a central piece, but no proximals. In all Devonian Crinoids 

 both plates are generally represented, but they do not attain their 

 full development until the Carboniferous. 



It has been asserted by us that the ventral plates in Allage- 

 crinus, Haplocrinus, Gvlicocrinus and Coccocrinus, are calyx 

 interradials and not proximals. In Allagecrinus and Haplo- 

 crinus there are five single plates occupying the same space as 

 the whole series of interradials in other genera. In their simpli- 

 city, and in resting upon the raclials and closing the peristome, 

 these plates, no doubt, closety resemble the orals in the Penta- 

 crinoid larva, but as calyx interradials they would occupy exactly 

 the same position. There is, however, a very important difference 

 in the structure of the two forms to which no attention has been 

 paid. The orals of the larva and those of Holopus rest loosely 

 upon the calyx ; while the interradials of Haplocrinus, like all 

 other interradials, are united with the raclials by a close suture. 



It has been proved from palgeontological evidence, that in the 

 earlier genera the interradials are more extravagantly developed 

 than in later ones. In Crotalocrinus and Reteocrinus, the inter- 

 radials cover the entire ventral surface ; in Glyptocrinus and 

 Glyptaster they extend to the central plate ; while in the Carbon- 

 iferous genera they recede gradually toward the periphery, and 

 the central space is filled by large proximals, and often by radial 

 dome plates. Considering these facts, is it safe to assert that in 

 Allagecrinus and Haplocrinus, which are regarded as larval 

 forms, interradials are entirely absent, and that all ventral plates 

 are actinal ? Is it not more reasonable to imagine that in these 

 low forms the ventral side was covered by the one plate, in a 

 similar manner as in Crotalocrinus, Reteocrinus and Glyptocrinus 

 hy the whole collection of plates ? In the Neocrinoidea, from 

 the larva to the adult, all ventral plates are actinal, but in all 

 Pakeozoic Crinoids, and we may say in all Palaeozoic Pelmatozoa, 



