204 PROCEEDINGS OF THE ACADEMY OF [1881. 



the3^ are indeed diflerent things, although there are certain analo- 

 gies between some of their parts. Among these are the oral plates, 

 which are represented in some of the later Crinoids, but absent 

 again in the fully grown PentacrinuH and in the Comatitlidse. 

 They are also absent in the Spluvroidocrinidte, unless we consider 

 the interradial vault pieces to be their representatives. We have 

 alread}' suggested that all intenadial plates in the dome — exclu- 

 siA'C of the proximal pieces — may perhaps have been modified 

 oral plates which, either by division or interpolation, gradually 

 increased in number. The dome of Coccocrinus has a single 

 oral plate to each interradial field, while the corresponding spaces 

 in most species of Plafyannuti are occupied by three and occa- 

 sionally five pieces each. It is as easy to consider the single 

 plate of the former to be represented by three in Platycrinus, as 

 that the three are sometimes replaced by five within the limits 

 of the same genus, the plates occup}^ the same jwsition in both 

 cases, but in some groups the true orals meet laterally which is not 

 the case with the interradial dome pieces of Platycrinus or Acti- 

 nocrinus, nor with the undivided plates of Coccocrinus. In 

 Cyathocrinus where the orals are very conspicuous, they join 

 beneath the radial groove, and form the floor upon which the am- 

 bulacral tube i-ests. The bottom of the tube is composed of two 

 series of pieces, which are covered directl}^ b}^ vault pieces in two 

 alternate rows, whose lateral margins rest upon the upper edges 

 of the two orals ; while in Platycrinus the corresponding vault 

 pieces abut laterally against the sides of the interradial — oral — 

 plates in an unbroken succession. In Platycrinus the interradial 

 plates thus take exactly the same position as the exposed part of 

 the oral plates in Cyathocrinus, while the covered parts are 

 unrepresented. In Coccocrinus^ a covering of the ambulacral 

 groove has not yet been observed, but judging from the fissure 

 between the oral plates, it probably rested just upon their edges, 

 and formed an intermediate link between the vault structure of 

 the Cyathocrinidse and Platycrinidfe. 



In the Actinocrinidfe and Rhodocrinidoe, the alternate dome 

 plates are not so readily distinguished, as in the Platycrinidfe 

 and forms with free rays, in which they are well marked in the 

 extended parts. In the recent Crinoids the alternate plates are 

 represented by the " Saumpliittchen," which, however, instead of 



