REPORT ON THE CRINOIDEA., 75 
noticed the great differences of form, size, and regularity of arrangement between the 
ambulacral plates of the arms and pinnules respectively. But he pointed out that the 
covering plates of the disk-ambulacra rest upon other plates which he called “ side plates,” 
and that both are distinguished from the general anambulacral plating of the disk by the 
absence of water-pores." It is difficult to individualise these plates when looking at the 
disk from above, as they are so irregularly arranged (PI. XVII. fig. 6; Pl. XX XIX. fig. 2 ; 
Pl. XLUI. fig. 3; Pl. L. fig. 2) ; but they are more easily distinguished in a cross section 
of an ambulacrum (Pl. LIV. fig. 11; Pl. LVII. fig. 3, sp.). 
Miiller further mentioned a series of median subambulacral plates as lying beneath 
the food-groove and water-vessel, which he believed to rest in a furrow along their upper 
surface ;” and he described a series of ambulacral pores between the median row and the 
side plates, which might be related to the tentacles, and possibly served for the passage 
of vessels connecting these organs with ampulle. He had previously figured some plates 
as underlying the sides of the food-groove, with pores in or between them, which he spoke 
of as “Oeffnungen des Tentakelcanals in die Tentakeln der Tentakelrinne.”* But it is 
difficult to make out whether they are identical with those which he subsequently 
described and figured as ambulacral pores.* 
In reality, however, there are no pores of this kind beneath the ambulacra of the 
disk; and there are no large ampullee connected with the tentacles as there are with the 
tube-feet of the Stellerids. But there is often a large amount of calcareous tissue beneath 
the water-vessels of both disk and arms, which takes the form of more or less regular 
plates (Pl. LIV. fig. 11; Pl. LVII. fig. 4, swb; Pl. LXII.).. They have no definite arrange- 
ment, however, and are practically only a portion of the general limestone plating beneath 
the upper surface of the disk. Although therefore, owing to their subambulacral position, 
they are generally equivalent to the rotulze of the Urchins, the lancet-plates of the Blastoids, 
and the radial pieces in the oral rmg of Holothurians, I do not think that they quite deserve 
the morphological importance which was attributed to them by Miiller. It is possible 
that the series of plates which were discovered by Prof. Huxley and described by 
Billings’ as forming an elongated arch beneath the subtegminal ambulacra of Actino- 
crinus rugosus may be true subambulacral plates. But from the descriptions of them 
which are given by Meek and Worthen,’ and also by Wachsmuth and Springer,’ I 
am rather inclined to think that they may be the adambulacral or side plates (Pl. LVIL. 
fig, 3, sp.). 
Besides going somewhat fully into the nature of the ambulacral skeleton in Penta- 
crinus asteria, Miller drew attention, as his predecessors had done, to the plates on the 
1 Bau der Echinodermen, p. 58. 2 Thid., pp. 57, 58, Taf. vi. figs. 7, 9, d. 
3 Bau des Pentacrinus, p. 70, Taf. ii. fig. 14. 4 Bau der Echinodermen, pp. 58, 63, Taf. vi. fig. 7, ¢. 
5 On the Cystidez of the Lower Silurian Rocks of Canada, Geol. Surv. of Canada, Decade iii, p. 27. 
6 Notes on the Structure and Habits of the Paleozoic Crinoidea, Paleontology of Ilinois, vol. v. p. 331. 
7 Revision, pt. ii. p. 28, 
