276 PROCEEDINGS OF THE ACADEMY OF [1885. 



be a doubt that the central piece, undivided as it is, is the true 

 horaologue of the oral pyramid as represented in the Penta- 

 crinoid larva. 



It is true that the pentamerous nature of the orals is an 

 objection to this interpretation, but we do not believe it a serious 

 one, certainly not so great as is found in attempting to homolo- 

 gize six proximals with five orals. 



Of the embryology of the Palseocrinoidea little or nothing is 

 known except from phylogenetic evidence, and even this is limited, 

 and gives information only as to the later stages in the almost 

 fully developed Crinoid. In recent Crinoids, from their earliest 

 stages, the orals are composed of five distinct plates, and it is 

 very possible that the central piece, if representing the orals of 

 the Palseocrinoidea, primitively consisted also of five pieces, 

 which were fused together, ami that the suture lines gradually 

 were obliterated by deposition of new material at the outer sur- 

 face, as in the case of the underbasals in Agassizocrinus. Who 

 would have thought that in this genus the thick plate at the 

 dorsal end represented five anchylosed plates, if the sutures had 

 not been fortunately observed in some of the younger specimens? 

 That a similar process probably took place at the outer face of 

 the central piece, is somewhat indicated by the condition of the 

 plate, which is alwa}'s more or less conical or spinil'erous, and 

 wherever the point of the plate has been broken, the missing part 

 is replaced by secretion of new deposit. It seems to us that in 

 a group like the Paloeocrinoidea, in which the tentacular vestibule 

 was permanently closed, a gradual anchylosis of the five primary 

 plates is deducible from analogy, and would be in entire accord- 

 ance with prevailing rules in nature. 



Such an ancl^dosis occurred in palaeozoic times among the 

 basals, and this is of considerable importance, as the basals are 

 admitted by Carpenter to represent the orals. In the larva of 

 Antedon, the basal ring is formed of live distinct plates, and the 

 same number prevails in the adult throughout the recent Crinoids, 

 if not throughout the Neocrinoidea generally. Among mono- 

 cyclic Pala?ocrinoids, however, this number forms the exception, 

 and occurs only in a few Silurian genera. Five are soon succeeded 

 by four, three and two plates. Carpenter finds no objection to 

 call all those plates basals, and to regard them, whether composed 

 of two, three or five plates, as the representatives and homologues 



