EOSACEUS, LINCK (COMATULA EOSACEA OF LAMAECK). 
537 
The centro-dorsal plate with its dorsal cirri in Antedon is therefore the homologue 
of the stem with its cirri in the stalked Crinoids. 
The true cup in the mature Antedon consists inferior] y of a delicate rosette of more 
or less fully coalesced small cribriform calcareous plates ; which have been shown by 
Dr. Carpenter, in a series of beautiful observations, to be the remains of the row of 
five basal plates which occupy so prominent a place in the cup of the Pentacrinoid. 
This rosette is completely concealed in the cavity of the ring formed by the first five 
radials. Around the basal rosette, and alternating with its segments, five elongated 
calcareous blocks, triangular in transverse section, the first radial plates, form a column 
within the base of the cup. In A. rosaceus these plates are entirely concealed by the 
centro-dorsal plate and by the series of second radials. In some species of the genus 
Antedon , they project beyond the centro-dorsal plate, forming above its upper edge a 
closed ring which supports the series of second radials. The centro-dorsal plate, the 
basals, and the first radials are immoveably cemented together ; they do not, however, 
coalesce, and may be easily separated after boiling in weak caustic potash. A ring of 
five second radial plates placed in close contact, form, externally, the base of the cup in 
Antedon rosaceus, resting within upon the upper surfaces of the first radials, and exter- 
nally upon the edge of the centro-dorsal plate. 
Kesting upon the second radials, we have next a row of five triangular axillary radial 
plates, each bevelled above into two diverging surfaces for the articulation of the first 
brachial joints. The axillary radials are not in immediate contact laterally, they are 
separated by minute wedge-shaped prolongations downwards of the perisom of the disk. 
In Antedon rosaceus, the basals, and the first, second, and axillary radials form the whole 
of the skeleton of the cup. 
In certain species of Antedon, as in A. Milleri (Muller, sp.), a series of five minute inter- 
radial plates are intercalated between the angles of the axillary radials, and in other 
forms, as in A. Solaris (Lam., sp.), and A. tessellatus (Muller, sp.), the whole of the 
perisom of the disk is covered with a pavement of irregular flat plates. We are unac- 
quainted with the development of Pentacrinus ( Neocrinus ) asterias (L.), but in the 
mature form the perisom of the disk is continuously tessellated, and some of the plates 
pass irregularly downwards between the axillary radials. In Pentacrinus ( Neocrinus ) 
decorus (nob.), the surface of the disk is rough with irregularly scattered blocks, like 
fragments of perforated bricks ; and these descend into the spaces between the axillary 
radials, though without any regular arrangement. 
The basal and oral plates. — The first portions of the skeleton which appear are the 
two rings of five plates each, the plates of the upper ring directly superposed on those 
of the lower, which form the trellised basket, completely enclosing the viscera of the 
Pentacrinoid during the early stages of its growth within the pseudembryo. The 
plates of the upper tier subsequently extend into the five oral lobes, and remain as five 
valve-like interradial oral plates during the greater part of the pentacrinoid stage. 
The lower series are the basals. These are permanent, with some remarkable modifi- 
MDCCCLXV. 4 D 
