THE AMBULACB \. 49 



separable from the lancet-plate as in Dr. I [hide's example of P. pyriform&s. lint 

 we are quite clear that neither of Ilambach's suggestions as to its nature is 

 anywhere near the truth. He intimates thai we may have been misled by " the 

 calcareous substance which is frequently found to fill out the duct" beneath the 

 lancet-plate. "We do not think this probable, as we have never seen the duct in 

 question. We have sometimes found a groove along the under surface of the lancet- 

 piece in other types (PI. XVII. figs. 1, 1 1), but we cannot make out that anything else 

 was contained in it besides the calcite usually found in the interior of the calyx. 



In the genera Mesoblastus, Granatocrinus, and Oryptoblastus then; is no under 

 lancet-plate. But the alternating folds and slits of the hydrospires are not thereby 

 exposed when the lancet-plate and side plates are removed, as would be the case in 

 Pentremites (PI. I. fig. 7; PL XII. fig. 13). For the hydrospire-slits are covered 

 up at each side of the ambulacrum by what we have called the " hydrospire-plate ' " 

 (PI. IV. fig. 4 ; PL VI. fig. 10 ; PL VII. fig. 15 ; PL VIII. fig. 20 ; PL IX. fig. 16; 

 PL X. figs. 11, 12, 14; PL XI. figs. 11-15). The inner wall of each hydrospire-sac, 

 i. e., that forming the boundary of the hydrospire-canal into which the lamellar tubes 

 open, is more substantial than in Pentremites, and it is produced upwards above the 

 openings of these tubes towards the ventral surface of the ambulacrum, until it 

 comes into contact with the sides of the radial sinus. This is very well shown 

 in the weathered specimen of Granatocrinus ellipticus which is represented on PL X. 

 fig. 14, the hydrospire-plate being distinctly visible between the lancet-piece and 

 the radials. This species has very narrow ambulacra, and the two hydrospire- 

 plates rise somewhat steeply, leaving only a narrow median slit between them, which 

 leads down into the interior of the calyx (PL X. fig. 12). The lancet-piece is 

 wedged in between them and so covers this slit. The same is the case in Granato- 

 crinus Perbiensis, except that the hydrospire-plates are even steeper than in G. ellip- 

 ticus, so that the cleft between them, at any rate at the tip of the ambulacrum, is 

 wider than in that species (PL XI. figs. 11-13). G. campanulatus is another form 

 of the same kind. The lancet-plate is broad, and supports the whole of the side 

 plates, so that hardly anything of the hydrospire-plate is visible at its sides (PL VIII. 

 fig. 15). But in Mesoblastus Bofei, M. crenulatus, M. elongatus, and Granatocrinus 

 Norwood! (PL IV. fig. 4 ; PL VI. fig. 10 ; PL VIII. fig. 6 ; PL X. fig. 11 ; PL XI. 

 fig. 14), the hydrospire-plates come up towards the ventral side much less steeply, 

 so that the lancet-plate is relatively lower. The consequence is, that the side plates 

 instead of resting entirely on the lancet-piece, as in Granatocrinus campanulatus 

 and 67. ellipticus (PL VIII. fig. 15 ; PL X. fig. 12), are partly supported by the 

 hydrospire-plate, which is moulded to receive them, just as the upper surface of the 

 lancet-plate is. This condition is especially well marked in Mesoblastus Jiofei, and 

 also in M. crenulatus, owing to the relatively small size of the lancet-piece and the large 

 amount of hydrospire-plate, which is thus exposed (PL IV. fig. 4 ; PL VI. tig. 10). It 

 1 Ann. & Mag. Nat. Hist. 1882, vol. ix. p. 215. 



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