STRUCTURE OF THE CYSTIDE^. 25 



of the arms, and that consequently it was not necessary for the 

 vessels to climb the proboscis in order to reach the interior. 



Fig. 4. Side view of a fragment of H. pristinus. 

 Fig. 5. The same, seen from above, shewing the continuation of 

 the ambulacral grooves of the arms into the interior. 



2. Hyhocrinus pristinus, H. conicus, and H. tumidus, (Billings,) have 

 five ambulacral orifices each, and they are formed according to a 

 plan which will be found somewhat common among the species of 

 those genera which have short cup-shaped or round bodies, such as 

 Cyathocrinus, Poteriocrinus, Dendrocrinus and others. The first radial 

 plate has a projection of the central portion of the upper margin, 

 which folds round and makes a conspicuous rounded channel which 

 descends along the inside of the plate to the interior. The upper 

 edge has a horse-shoe form, corresponding exactly to the first joint 

 of the arm which is seated upon it. These species shew that 

 generally the notches which we see in the detached first radial 

 plates of so many others are only continuations of the grooves of 

 the arms into the interior, 



3. Rhodocrinus hursa (Phillips). — Good specimens of this species 

 exhibit very distinctly ten ambulacral openings. They penetrate into 

 the interior at about one-half the height of the body, and their margins 

 are formed on the lower sides by a semi-circular notch in the upper 

 edge of the second plate of each of the secondary rays, and on the 

 upper by several of the small abdominal plates. In no other 

 species is there more unequivocal evidence of the existence of these 

 openings, but they are accompanied by a structure which seems to 

 indicate two sets of arms placed one above the other. Beneath the 

 orifices there are two articular surfaces, which mark the bases of 

 two arms ; and above each pair of the orifices there is a projection, 

 which also much resembles the base of one or two more arms. 

 They are very accurately figured in Phillips' " Geology of Yorkshire," 

 vol. ii. pi. V. figs. 23, 24, 25. I shall introduce one of these figures 

 here, in order to shew their peculiar structure : — 



