114 THE CRINOIDEA CAMEEATA 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 Cyathocrinidse, in which it was probably more substantial than in 

 the other families. In Cyathocrinus it is well shown in our figures on Plate 

 in. 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 Eoemer 

 and others in Ciipressocrinus. 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. Batlier is of the opinion that the ventral sac of the Fistulata is not perforated, but only pitted 

 (Geol. Magazine, May, 1891, p. 233, and elsewhere). This may be true as to Cyathocrinus, JEuspirocrinus, 

 and possibly the CyatliocrinidcB generally, in which very likely the madreporite performed the functions of the 

 tube-pores ; but we have the most complete evidence that among the Poteriocrinidse, in many cases, 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. Pigs. Si^, 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. 



