1887.] NATURAL SCIENCES OF PHILADELPHIA. lOt 



take the place of the single one at the two other sides. Like most 

 of the Cystidea, Caryocrinus has no true radials, although it has 

 well developed arms. The rays start from underneath the central 

 plate in a similar manner as they do in allied genera from under- 

 neath their quinque-partite oral 2:)yramid; but the ambulacra, instead 

 of entering the surface at once, as in other groups, here remain sub- 

 tegminal until they enter the arms, following the medium line of 

 three radial plates, and branch (fig. 7) underneath them twice to 

 their respective arm openings. In this case, the central piece which 

 "covers the mouth and the origin of the ambulacra" must surely 

 represent the orals if any plate does, but not the plates Avhich sur- 

 round it and cover neither the mouth nor the origin of the ambu- 

 lacra. We should like to know by what process Messrs. Etheridge 

 and Carpenter will demonstrate the oral nature of either the radial 

 or interradial plates in this form. 



We have already alluded to the great importance toward the estab- 

 lishment of Etheridge and Carpenter's oral theory, of their proving 

 the existence both in Crinoids and Blastoids of a summit composed 

 either of five plates only, or of a central plate surrounded by five. 

 This is why the series of jaarallel transitions or variations in the 

 summit plates of the two groups is so strenuously urged in the 

 Blastoid Catalogue. But it seems to us that the authors have al- 

 together failed to point out a single instance in which five primary 

 plates cover the peristome among the Blastoids. The cases which 

 have been relied upon to prove such a condition, must be attributed 

 to incorrect observation or the want of sufficiently good material. 

 That occasionally in certain species of Elaeacr'mus the central piece 

 is more or less coalesced with the proximals of the azygous side, and 

 these with one another so as to obscure the suture lines, as we have 

 shown in our illustrations figs. 12 to 14, cannot alter the case in 

 the least, as they are plainly visible in others, and without any 

 change in the general arrangement of the summit. Nor does it seem 

 to us that the authors have been any more successful in showing 

 how among Palaeocrinoids the five large plates in Haplocrinus, cov- 

 ering the whole ventral surface except the oral pole, could have 

 been transformed in other groups of the Palaeocrinoidea into six 

 plates covering only a small space around the peristome. These five 

 plates in Haplocrinus occupy the same position, as the primary calyx 

 interradials of other groups, and especially resemble those of 

 Cyathocrinus and Stephanoerinus. (Compare figs. 2 to 5). 



