212 PROCEEDINGS OF THE ACADEMY OP [1886. 



a satisfactory conclusion as to the morphological resemblance of 

 the respective plates. 



In Stephanocrinus interradials have been admitted, and the 

 plates which do represent them correspond in our hj-pothetical 

 Crinoid with the plates which we opened out to receive the ambu- 

 lacra. The plates have relativel}^ a similar position, both rest 

 against the upper edges of two adjoining radials, and both support 

 a small pyramid or disk, which in both cases not only occupies a 

 strictl}' central position, but covers the peristomial area and closes 

 the oral pole. Now, if this is true, what makes those plates inter- 

 radials onl}^ in the one case and not also in the other, and why does 

 the central disk which they enclose represent the oral pyramid in 

 the one and something else in the other? That the central pyra- 

 mid is quinque-partite in Stephanocrinus, coalesced in the other, 

 is apparent!}' the only structural difference between the two forms, 

 and simply upon this ground the former has been regarded by 

 Carpenter (Chall. Rep., pp. 269 to 271) an oral pj'iamid, and the 

 central plate of Haplocrinus a so-called "orocentral," something 

 totally unknown in Crinoid morphology and that of Echinoderms 

 generally. 



According to Carpenter the orals of Haplocrinus were repre- 

 sented by the five large ventral plates, although these, like the 

 interradials of other Paloeocrinoids, apparently cover the disk and 

 tentacular vestibule, contrary to the case of the orals and summit 

 plates generally which close only the peristomial area. If Haplo- 

 crinus did represent a permanent larval form of the Neocrinoidea 

 instead of the PaltEOcrinoidea. Carpenter would be justified in re- 

 garding those five plates as orals, and could assert that the plates 

 were in the growing animal, and in palaeontological times carried 

 inward b^^ perisome, as he and Etheridge suggested to have been 

 probably the case in the growing Allagecrinus, but we cannot find 

 in the phylogeny of the older Crinoids the least evidence to justify 

 that supposition. On the contrary, everything points at the con- 

 clusion that the orals, and other summit plates, had relatively the 

 same proportions in the younger and lower forms, as in the adult 

 and higher tj^^es ; and we, therefore, regard the respective plates 

 in Haplocrinus like those of Sfephanocrinus,Cyatlwcrinus and the 

 Blastoidea, which Carpenter, likewise, once regarded as orals, as 

 true interradial plates. If the plates in Haplocrinus really were 



