92 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
plates by the letters “rx” in the accompanying diagrams.* There now re- 
mained among the plates of the proximal ring but four which could possibly 
be taken for the orals; and this led us to inquire whether the central plate 
alone might not be a coalesced representative of the five orals of recent forms. 
From internal casts we observed that this plate occupies the centre of 
radiation, and that not only the ambulacra, but also the nerve cords, meet 
beneath it. It was this structure principally which led us to the assumption 
that the central plate represented the five orals collectively, and that the 
four large proximals, and two smaller ones, were interradial “ vault” plates, 
corresponding with the first interradials of the abactinal side. This seemed 
to us the more probable, as in the dorsal cup a division of the first interra- 
dial into two halves by an anal plate is a frequent occurrence among Palzxo- 
zoic Crinoids. It also seemed to explain why in /aplocrinus and allied forms, 
in which there is no anal plate, the central piece seemed to be surrounded 
by five plates instead of six, supposing, as before stated, that Haplocrinus had 
a small oral surrounded by five interradial plates, and Ad/agecrinus, Coccocrinus, 
and Culicocrinus five interradials, but no orals. In this we differed from Gitte, 
Carpenter, Zittel, and Neumayr, who all agreed that the Scheitelplatten were 
the orals.t 
This was the state of the question in 1888, when we came into posses- 
sion of a very large number of fine specimens of Haplocrinus mespiliformis 
in various stages of preservation, and sound to our astonishment that 
such a thing as a “central” plate does not exist in the genus. We now 
saw that the ventral disk consists of but five large plates; that we had 
mistaken a mere fracture for a suture; and that the part which we sup- 
posed to be a separate piece was a tongue-like prolongation of the posterior 
plate, projecting in between the other four plates, and sometimes surmounted 
by a small node (Plate TII., Fig. 124). This discovery left no room for doubt 
that the large ventral plates of Haplocrinus, and of the Laviformia generally, 
actually represent the five plates composing the unopened oral pyramid of 
the Pentacrinoid larva before it moved away from the radials, as had been 
contended by Carpenter and Geette. 
So long as a central plate was recognized in Japlocrinus, we saw good 
reason to believe in the existence of a similar plate in other groups of the 
* Revision, Part III. Plate VIT. Figs. 2, 3, 4, 5, 6, 8, 9, 10, and Plate VIII. Figs. 1 and 2. 
¢ Our theory of the relations of the summit plates, in conformity with these views, was discussed in the 
Revision, Part III. pp. 44 to 59, and afterwards in greater detail in our paper on the Summit Plates, in the 
Proceedings of the Academy of Natural Sciences, Philadelphia, March 29, 1887. 
