116 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
the orals; but while those of the Cyathocrinide enclose a madreporite, those 
of the Blastoids enclose an anal piece. The conditions are thus not exactly 
alike, and we do not see why that term is any more appropriate than “ in- 
terradials ’’ —a term which has the advantage of being more general. 
In the Hybocrinidz the posterior side of the disk is less elevated than in 
the Cyathocrinide, rising but little above the level of the other four sides. 
The ambulacra are tegminal, and apparently rest upon the lateral margins 
of the four large interradial plates. There are no orals, the median portions 
of the disk being occupied by large covering pieces. 
The ventral pavement of the Anomalocrinide, which we examined in 
a fine specimen from the Museum of Comparative Zodlogy, differs essentially 
from that of the two preceding families. It consists of rather large, moder- 
ately thick, irregular pieces, five or six deep, which decrease in size toward 
the centre and cover the peristome; no orals being distinguishable. The 
outer of these plates rest against the large incurved limbs of the radials, 
forming a wide and deep sinus for the reception of the ambulacra, which are 
on a lower level than the other disk plates, and in the Cambridge specimen 
of a different color, and thereby readily recognized. 
The tegmen of the Poteriocrinide has never been found in perfect pres- 
ervation. We hoped to find it by dissecting a number of finely preserved 
specimens, but the sacrifice was only rewarded by finding in a single instance 
a few small, scattered plates; from which we conclude that the covering was 
of very fragile construction. The form and size of their ventral sac is ex- 
tremely variable; extending in some cases beyond the tips of the arms, in 
others consisting of merely a short cone. In some of the genera it is tubu- 
lar, in others club-shaped; in some balloon-shaped ; in some even spiral, the 
whorls of the coils either united or free; but in all cases, so far as we know, 
the plates are arranged in vertical rows, which sometimes diverge at intervals, 
and apparently in all of them the edges of the plates — not their substance 
—are perforated by pores or clefts. 
A porous sac is also found in the Belemnocrinidx, and probably exists in 
the Heterocrinidx, but nothing is known of the other disk plates in these 
families. 
In the Encrinide, according to Wagner,* the ventral disk rises to about 
the height of the second costals, where it contracts abruptly to one half its 
diameter at the base, and is surmounted by a small cone. The peripheral 
* Zeitschr. d. Deutsch. Gesellsch., 1887, pp. 822 to $28. 
