94 THE CEINOIDEA CAMEEATA OF NOETH AMEEICA. 



of the others, and almost twice as long as wide, extending well in between 

 the two posterolateral 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 Taxocrimis 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 ^^^^ plates of this specimen, although somewhat unequal in size, 

 represent morphologically the ^^q orals of the recent genera Bhizocrinus, 

 Hi/ocrmus, and Holopus, nobody will deny after seeing the specimen. And a 

 comparison of these plates with the so-called central plate and four large 

 proximals in FMycrimis^ Actmocrinus, etc., 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 Taxocrinus, 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 III. When the anus is excen- 

 tric, and its plates are small, the arrangement of the orals is comparatively 

 symmetrical j 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 Haplo- 

 criniis, in which the anus penetrates the orals, and in Coccocrimis, 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. 



