338 PROCEEDINGS OF THE ACADEMY OF [1888. 



and it was this, principally, that led us to the supposition that the 

 central plate, and this only, represented in the vault of the Palseocri- 

 noids the five orals collectively, and that the four large and two 

 smaller proximals were interradial vault plates, corresponding to the 

 interradials of the abaetinal side (Rev. III., pp. 44-59). The latter 

 was contrary to the views originally expressed by us (Kev. II, pp. 15 

 and 16), when we supposed that "the six proximals surrounding 

 the central plate represented the basals or genitals." The gi'eat 

 objection to this interpretation was that it involved a homology 

 between six plates and five, and we were so greatly impressed with 

 the force of it, that we wei'e afterwards led to consider these plates 

 as interradials, as to which on the dorsal side a division of the pos- 

 terior interradial into two plates by the interposition of an anal 

 plate is a frequent occurrence in Pala^ocrinoids. It seemed to us 

 therefore very natural that a similar division of the posterior plate 

 should be found on the ventral side. 



Dr. P. Herbert Carpenter, like ourselves, recognized a central plate 

 and six proximals, but he regarded the former as the actinal representa- 

 tive of the dorso-central or terminal plate of the column in the Peii- 

 tacrinoid larva, and established for it the term "oro-central," as a 

 distinct element in the vault of the Palaiocriuoids, unpresented in 

 other Echinoderms. He adopted the theory that the surrounding 

 six proximals are the homologues of the basals, and as such are the 

 oral plates — he considering that the posterior one was divided by 

 anal plates into two. His views on this subject are fully set forth 

 in the Challenger Report on the Stalked Crinoids, pages 158 to 184, 

 and the same interpi-etation of the plates in question was reasserted 

 by Etheridge and Carpenter in the Catalogue of the Blastoidea in 

 the Geological Department of the British Museum, pages 66 to 75. 



Although this conception of the morphological relations of the 

 proximals agreed with the ideas we originally entertained, as before 

 mentioned, we found ourselves unable to reconcile it with the diffi- 

 culty arising out of a homology of six plates which surround but do 

 not cover the oral center, with a set of five closed oral plates which 

 cover the mouth. This objection did not exist as to the central 

 plate which covers the oi'al center, and it seemed to us, therefore, 

 more reasonable to regard that plate, though undivided, as the re- 

 presentative of the five orals, than to consider it an entirely new ele- 

 ment in Echinoderm morphology, which the so-called "oro-central" 

 of Carpenter certainly was. Our theory of the relations of the sum- 



