KEPORT ON THE CRINOIDEA. 75 



noticed the great differences of form, size, and regularity of aiTangement between the 

 ambiilacral 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. XYII. fig. 6 ; PI. XXXIX. fig. 2 ; 

 PL XLIII. fig. 3 ; PI. L. fig. 2) ; but they are more easily distinguished in a cross section 

 of an ambulacrum (PI. LIV. fig. 11; PI. 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 ampullae. He had previously figured some plates 

 as underlying the sides of the food-groove, with j)ores in or between them, which he spoke 

 of as "Oefiuungen 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 ampuUse 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 (PI. LIV. fig. 11 ; PI. LVII. fig. 4, sub; PI. LXIL). 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 rotulte of the Urchins, the lancet-plates of the Blastoids, 

 and the radial pieces in the oral ring 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 

 Billinss ^ as formins: an elongated arch beneath the subtegminal ambulacra of Actino- 

 criims rugosus may be true subamljulacral 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 LVII. 

 fig. 3, sp.). 



Besides going somewhat fully into the nature of the ambulacral skeleton in Penta- 

 crinus asteria, Miiller drew attention, as his predecessors had done, to the plates on the 



1 Bau der Echuiodernien, p. 58. " Ibid-, pp. 57, 58, Taf. vi. figs. 7, 9, d. 



3 Bau des Pentacrinus, p. 70, Taf. ii. fig. 14. * Bau der Echinodermen, pp. 58, 6.3, Taf. vi. fig. 7, e. 



5 On the Cystidea- of the Lower Silurian Bocks of Canada, Geol. Surv. of Canada, Decade iii. p. 27. 



^ Xotes on the Structure and Habits of the Paleozoic Criuoidea, Palitontology of Illinois, voL v. p. 331. 



' Revision, ^rt. ii. p. 28. 



