94 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
of the others, and almost twice as long as wide, extending well in between 
the two postero-lateral plates. 
The positions of these plates are relatively identical with those of the five 
plates at the summit in certain forms of Platycrinus, such as are illustrated on 
Plate III., Figs. 16 and 17, except that the plates of Zaxocrinus do not meet 
in the centre, but leave a slightly excentric, obtusely pentagonal oral open- 
ing, transversely elongated, its longest side next to the posterior plate. Into 
this opening, which is deep, the ambulacra converge, and turn downwards at 
the five corners. 
That the five plates of this specimen, although somewhat unequal in size, 
represent morphologically the five orals of the recent genera hizocrinus, 
Hyocrinus, and Holopus, nobody will deny after seeing the specimen. Anda 
comparison of these plates with the so-called central plate and four large 
proximals in Platycrinus, Actinocrinus, ete., leaves no room for doubt that these 
are likewise true orals. The arrangement of the plates in the different 
groups is practically the same ; the only difference is that in some they are 
less symmetrically disposed than in others. In 7Zuzocrinus, the anus is well 
removed from the oral centre, which accounts for the fact that the arrange- 
ment of its orals is rather more symmetrical than in most of the Camerata, 
in which the anus is more or less subcentral, and surrounded by heavy, rigid 
plates. That the amount of asymmetry depends upon the condition of the 
anus is clearly shown by the figures on Plate II]. When the anus is excen- 
tric, and its plates are small, the arrangement of the orals is comparatively 
symmetrical ; but when it is subcentral, and especially when it is extended 
into a large tube, the orals are pushed over to the anterior side. In Hapl- 
erinus, in which the anus penetrates the orals, and in Coccocrinus, in which it 
occupies the arm regions, the oral pyramid is naturally about symmetrical. It 
is now easy enough to understand how a set of five plates, symmetrically dis- 
posed over the mouth, could be so altered by the introduction of anal plates 
as to bring the mouth and centre of radiation beneath the posterior plate. 
By the encroachment of the anal plates the posterior oral was pushed to a cen- 
tral position, and thereby the mouth came to be placed beneath that plate. 
The above explanation of the orals in the different groups met the 
prompt approval of Carpenter, and this ended a long controversy which 
had been going on between us for over six years. The orals were found 
at last to consist of four of the proximals which he had claimed, with the 
addition of the so-called central plate which we had contended for. 
