90 CATALOGUE OP THE BLASTOIDEA. 



the side plates and deltoids. Thus, for example, there are ten in the original of 

 PI. XIII. fig. 20, one on either side of each oral ridge, as in Pcntremites Godoni 

 (PI. I. fig. 11); while the specimens shown on PI. V. figs. 23, 24 have but five 

 openings, as in the ordinary Pcntremites. A vertical section of the hydrospires of 

 Cryptoschisma Schuhi is seen in PI. XVIII. fig. 2 ; while figures 3 and 4 on 

 PI. XVI. show their distal termination against the inner faces of the radials. 



The transition between the two chief types of hydrospires in the Blastoids is 

 presented by the genus Orophocrinus. Those of the Belgian species, 0. Orbignyanus 

 and 0. Puzos, are constructed on the type of Codaster and Cryptoschisma; while 

 some of the hydrospires in the English species, 0. vents and 0. pentangularis, have 

 nearly the same relation to the calyx-plates as those of Pcntremites (PI. I. figs. 6, 7 ; 

 PI. XII. fig. 13). The two Belgian species are somewhat aberrant members of the 

 genus, and but for the form of the calyx and the appearance of the deltoid plates 

 externally they might well be referred to Phcenoschisma. In 0. Puzos one at least 

 of the hydrospire-slits is visible on the side of the radial sinus above the ambulacrum 

 (PL XIV. figs. 14, 15), and the latter, though fairly wide, is not sufficiently so to 

 come into direct contact with the sides of the sinus, so that a cleft appears between 

 the side plates and the radials which the former do not bridge over ; and it leads 

 downwards beneath the edge of the ambulacrum to the bottom of the radial sinus. 

 As the side plates never meet the deltoids this hydrospire-cleft is nowhere converted 

 into a canal, so that no spiracles are visible at its proximal end. Its nature is more 

 clearly shown in 0. Orbignyanus (PL XIV. figs. 16-18). This species varies a good 

 deal in the amount of exposure of the hydrospire-slits. Thus, for example, on one 

 side of the radial sinus represented on PL XIV. fig. 18, two slits are left uncovered 

 at the sides of the ambulacrum ; while both proximal and distal ends of a third are 

 also visible, very much as in Phcenoschisma Archiaci (PI. XIV. fig. 6). But only 

 one slit is completely visible on the other side, while the slits of the sinus represented 

 on PL XI. fig. 10 are almost entirely covered by the wide ambulacrum, which, however, 

 does not touch the sides of the sinus ; for it leaves an intervening hydrospire-cleft 

 which passes downwards above the slits, as is better seen in PI. XIV. fig. 16. In 

 the remarkable species Orophocrinus pentangularis, which is common to Belgium and 

 to Britain, the radial sinuses are much reduced in width as compared with those of 

 0. Orbignyanus (PI. XIV. fig. 16 ; PL XV. figs. 5, 8) ; and their sides are tolerably 

 steep, though rather low, owing to the comparatively horizontal position of the 

 ambulacra, which does not necessitate any deep incision of the rather widely spread 

 radials. The hydrospire-slits are consequently not situated at a little distance apart 

 on the sloping sides of the radial sinus as in 0. Orbignyanus and in Phcenoschisma 

 (PL XI. figs. 1-6, 10 ; PL XIV.) ; but they are crowded together at the sides of the 

 ambulacra at the bottom of the sinuses (PL XV. fig. 10), very much as in Pentremites 

 (PL I. figs. 6, 7). The section shown in PL XVII. fig. 14, however, indicates tint 



