114 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
Essentially different is the ventral structure of the Fistulata, which have 
no interradial plates in the dorsal cup, the anal piece excepted; but those 
plates are extensively developed in the tegmen. Four of the interambu- 
lacral spaces are raised but little above the level of the arm bases; while the 
posterior area is extended abruptly upward, and formed into a sac or tube of 
various forms, frequently rising beyond the tips of the arms. This sac, 
which may be regarded as a greatly extended anal area, and probably 
lodged a large portion of the visceral mass, must not be confounded with 
the anal tube of the Camerata, which contains simply the rectum. The 
sac is tubular, balloon-shaped, spiral, or club-shaped, and is generally com- 
posed of longitudinal rows of hexagonal plates, which are often pitted at 
their sides, or perforated by pores.* 
The structure at the four other sides of the disk is rarely observed ex- 
cept in the Cyathocrinide, in which it was probably more substantial than in 
the other families. In Cyathocrinus it is well shown in our figures on Plate 
III. Figs. 1, 2, 3, 4, 6, 7, 8, to which we drew attention in our chapter on 
the orals. There are six plates, interradially disposed, resting against the 
inflected upper edges of the radials, their lateral margins covered by the 
ambulacra. Four of them are larger, cordiform, and of equal size; the two 
others, which lie at the posterior side, are quite narrow, and enclose a large 
perforated plate to which we have applied the term madreporite. The sur- 
faces of the four larger plates in perfect specimens are roofed over by num- 
erous delicate pieces; while the perforated plate is completely exposed 
to view. 
As to the relations of these plates various conflicting views have been 
expressed. Wachsmuth, in 1877, supposed that the plates collectively con- 
stituted a kind of consolidating apparatus, like that described by Roemer 
and others in Cupressocrinus. We afterwards, supposing that there were five 
plates, suggested that they were structurally identical with the deltoids of 
the Blastoids, which at that time were regarded as homologous with the orals 
of the Crinoids. Similar views were expressed afterwards by Carpenter, 
* Mr. Bather is of the opinion that the ventral sae of the Fistulata is not perforated, but only pitted 
(Geol. Magazine, May, 1891, p. 223, and elsewhere). This may be true as to Cyathocrinus, Euspirocrinus, 
and possibly the Cyathocrinide generally, in which very likely the madreporite performed the functions of the 
tube-pores ; but we have the most complete evidence that among the Poteriocrinide, in many eases, the pores 
pass through the test. In some specimens in which the lateral edges of the plates were exposed perfectly 
free from the matrix, we have been able to trace the ducts of the pores to the full depth of the plates. They 
are well shown on Plate VII. Figs. 24, 5, 7, 8,9. That the openings are not visible in most of the speci- 
mens is due to their small size and probably also to the mode of petrifaction. 
