48 CATALOGUE OF THE BLASTOIDEA. 



part, just as we have found to be the casein one form of Pentremites pyriformis (PI. I. 



figs. 6, 7). As in that species too, the under lancet-plate is not wide enough to cover and 

 conceal the hydrospire-slits at the proximal end of the radial sinus ; though it comes 

 into contact with the radials for about one third of its length, just as is the case, 

 though in a greater degree, in Orophocrinits stelliformis (PI. XV. fig. 13). On the 

 other hand the under lancet-plate of Orophocrinus pentangularis, so far as can be 

 judged from the fragment of it which remains, is narrow and linear, not meeting the 

 radials, and so hardly covering the hydrospires at all (PI. XV. fig. 10). 



We do not think therefore that the presence of an under lancet-plate in Oropho- 

 crinus will be doubted any longer. That of Pentremites is much thinner and 

 altogether less substantial, a fact which may have misled Ilambach into regarding it 

 as " the upper blade of the hydrospiric sac." It is quite true that this does appear 

 to be the case in specimens like the P. Godoni represented in PL XII. fig. 16, in 

 which the under lancet-plate extends right across the radial sinus, and completely 

 conceals the hydrospires. But this explanation is useless for those forms in which 

 this plate is narrow relatively to the ambulacra, so that the hydrospires are visible at 

 its sides as in P. piriformis (PI. I. figs. 6, 7 ; PI. XII. fig. 13). These species are 

 in the same relation to P. Godoni as Orophocrinus pentangularis to 0. stelliformis 

 (PI. XV. figs. 10, 13) ; and if Hambach be right in regarding the under lancet-plate 

 of Pentremites as merely " the upper blade of the hydrospiric sac," this sac must be 

 open above in P. piriformis along a considerable length of the ambulacrum ; but 

 the section represented on PI. XVIII. fig. 3 shows that this is not the case. Then, 

 again, the under lancet-plate is essentially a mid-ainbulacral structure, occupying the 

 median line of the radial sinus; but if, as supposed by Hambach, it is the upper 

 blade of the hydrospiric sac, it should appear as a double plate with a long slit 

 between its two halves leading down into the cavity of the calyx, such as is actually 

 present in Granatocrinus ellipticus (PI. X. fig. 12). For the hydrospire-folds on the 

 two sides of the ambulacrum do not always meet one another so closely as is repre- 

 sented in Hambach 's " transverse section of a restored ambulacral field " '. Thus for 

 example in all the five species of Pentremites in which we figure the hydrospires, 

 and also in Wachsmuth and Springer's section of P. piriformis ", there is a more or 

 less wide gap between the innermost lamellar tubes on the two sides of the 

 ambulacrum (PI. XVI. figs. 19, 20 ; PI. XVIII. figs. 3-0). This is well marked in 

 both our varieties of P. p>>jriformis, in which the under lancet-plate is very clearly 

 visible in the median line of the sinus (PI. I. fig. 7; PI. XII. fig. 13), though, 

 from Hambach's explanation of its nature, it lias no business there. Unlike 

 Wachsmuth and Springer, we have failed to see the under lancet-plate in transverse 

 section, not having been able to cut any other specimen in which it is so distinctly 



1 Trans. St. Louis Acad. Sii. 1SS4, vol. iv. no. 3, p. 538. 



2 " Itevision of the Pakeocrinoidea," Tart L. pi. iii. fig. 5. 



