1887.] NATURAL SCIENCES OF PHILADELPHIA. Ill 



between them and the radials was naked, as in Bhizocrinus, or plated, 

 as in Hyocrinus, must of course remain undiscovered." 



This explanation is suggestive enough of what may occur in the 

 Neocrinoidea, but they fail to give a parallel case in which such a 

 development as this took place in a single Palaeocrinoid, and this 

 omission is the more important since they place the genus Allagecri- 

 nus in the latter group. They state afterwards (op. cit. p. 289). "It 

 is true we have no proof that there were any orals at all in the older 

 specimens; but, judging from the relative sizes and development of 

 the largest examples with oral plates, and the smallest without, we 

 think it scarcely likely that they were entirely unrepresented in the 

 adult. It is obvious that, if they were united to the radials by peri- 

 some, whether plated or bare, they would be readily lost under con- 

 ditions that would have had no destructive effect on younger speci- 

 mens, in which there was a closer union between the two rings of 

 plates." 



From the foregoing quotation, it is obvious that the Authors de- 

 sired to prove from the fact that the ventral plates were not found 

 preserved in what they regarded as the most mature stages of the 

 species, that they could not have rested upon the radials as in their 

 younger examples, and that they were parted from the radials by 

 perisonie. Upon this proposition we will observe that we have never 

 found among Palaeocrinoids the slightest evidence indicating to us 

 that any of the summit plates were carried inward by perisome. We 

 find that among the Camarata they occupy a comparatively small 

 space, but larger than in the Blastoidea, and that in all cases in 

 which they occur, they are supported by the upward growth of the 

 interradials. In the simpler forms of the Inadunata, when observed, 

 they rest upon a single interradial plate as in the case of the Siluri- 

 an Cyathocrinus alutaceous (Aug.). In the Carboniferous form of 

 Cyathocrinus, in which the ambulacra are placed upon the lateral 

 edges of the interradials, the orals are not carried inward by 

 perisome, but the perisome appears upon the surface of the interra- 

 dial plates. That the ventral plates were not preserved in the so- 

 called adult specimens of Allagecrinus is no proof that they did not 

 exist, or that they were carried inward. The simple fact that the 

 radials underwent the change from the horse-shoe form to a higher 

 state of development, having strongly marked articular facets, ex- 

 tending to the whole width of the plates, is sufficient to explain Avhy 

 the interradials were not intact or reduced in the adult stages. We 



