EOSACEITS, LINCK (COMATTJLA EOSACEA OE LAMAECK)/ 539 
cribriform plate which makes its appearance at the same time close to the posterior 
extremity of the pseudembryo, behind and below the pseudoproct. Immediately beneath 
the basal plates an irregular calcareous ring is early formed, considerably wider and 
broader than the ordinary rings of the stem. This ring, which is subsequently deve- 
loped into the permanent centro-dorsal plate, gradually thickens and becomes more 
regular in form, maintaining its position at the top of the stem, the lower edges of the 
basal plates resting on its upper surface. During the earlier stages of the growth of the 
pentacrinoid it is simply a circular band of the ordinary calcified areolar tissue, enclosing 
a sheaf of the peculiar fasciculated tissue of the stem, gradually enlarging, with a central 
aperture continuous with the bore of the tube-like stem-joints. It is not till some time 
after the latest stage described in the present memoir, that the rudiments of the first 
dorsal cirri appear round its lower contour. The rings which originate the ordinary 
stem-joints commence as small curved hollow spicules. At first they may often be seen 
open and imperfect ; afterwards they completely close (Plate XXIV. fig. 6). The inner 
surfaces of the rings are smooth, the outer roughened with projecting branches. I have 
only once or twice seen the rings of the stem in this early simple stage. Very soon after 
their appearance, usually before the pseudembryo has attained its full size, a hollow 
sheaf of calcareous rods united by minute calcareous trabeculae arises within each ring. 
The stem-joint increases in length by additions to each end of these cylinders. The 
centre of the cylinder is occupied by a consistent sarcodic thread running through the 
whole length of the stem. At this stage no fibrous tissue can be detected, either mixed 
with the calcified tissue or in the outer perisom. Additions are made to the length of 
the stem by the formation of new rings immediately beneath the centro-dorsal plate, the 
new rings becoming, as in the former case, gradually filled up by cylinders of linear cal- 
cified tissue. As the calcareous axis of the stem increases in width, the original rings 
girding the centre of the joints expand. They remain permanent during the whole of 
the fixed stage, and give the stem of the Pentacrinoid its characteristic beaded appear- 
ance. The terminal plate of the stem is formed on the same plan as the basals and 
orals. It is developed as a simple round cribriform plate within the posterior extremity 
of the pseudembryo ; and when this extremity becomes expanded into a disk of attach- 
ment, it supports and forms the skeleton of the terminal sucker. Afterwards it becomes 
thickened by irregularly deposited calcareous matter. The layer of soft tissue between 
the calcareous disk and the point of attachment seems to be at length absorbed, and the 
stem is permanently fixed by amorphous cement. 
The first and second radial joints and the axillary radials . — Shortly after the fixing 
of the Pentacrinoid and the opening of the cup, a third series of five plates make their 
appearance as minute branching spiculse occupying the spaces left by the bevelling off 
of the upper angles of the basal plates and the lower angles of the orals, thus forming 
an intermediate series between the basals and orals, and alternating with them. The 
spicula indicating the origin of these plates, the first radials, branch and extend in the 
manner already described, till at length they form diamond-shaped films consisting of a 
4 d 2 
