242 J. Bofe — Notes on the Crinoidea. 



stones/ so common in the Chert of Derbyshire, there is good reason 

 to believe, are in reality siliceous pseudomorphs of this membrane, 

 the calcareous part of the column having been entirely dissolved and 

 removed. (See Plate YI, Fig. 8.) 



To test this belief, some portions of a Pentacrinus caput-meduscB 

 ■were procured, and a few joints of the column gently boiled for 

 some minutes in a test tube with liquor potassa, when the membrane 

 was dissolved ; some of the plates fell asunder, and the others were 

 easily separated, whilst warm, by the point of a needle. The 

 figures formed by the articulating surfaces are beautifully defined, 

 as shown in Fig. 1, where the five radiating elliptical lobes a, a, are 

 well seen. The central aperture, generally called the alimentary 

 canal, is surrounded by a ring, in which there is seen a circle of 

 minute tubes,^ indeterminate in number. The interspaces between 

 the lobes are of reticulated, or, perhaps more correctly, of vesicular 

 structure. The lobes are similar in structure, but are pierced with 

 innumerable small tubes or pores, through which pass the fibres 

 below described. This experiment leaves the calcareous portion of 

 the joints of the column in a state similar to those separated by the 

 natural decomposition of the membrane, and found fossil, in many 

 limestones, such as the Saint Cuthbert's beads from Holy Island. 

 Having thus separated the calcareous part, to exhibit the membrane, 

 another portion of the column (which had been accidentally frac- 

 tured longitudinally, but leaving the central opening and the whole 

 of three lobes perfect) was slowly decalcified by very dilute muriatic 

 acid, occasionally adding more acid when required, as shown by the 

 cessation of chemical action, until the decalcification had gone as 

 far as it was prudent in this instance to carry it, as it was doubtful, 

 if the whole of the lime were removed, whether from the fracture 

 the fibres would not have collapsed. Very shortly after chemical 

 action commenced bundles of very fine fibres (above alluded to), 

 and insoluble in the acid, were visible, projecting from the lobes 

 a, a, at both ends of the specimen, and the fibres were visible in the 

 broken lobe the whole length of the fracture. Figure 6 shows the 

 fibres from one of the broken lobes, the cross lines being the 

 membrane between the calcareous plates. This membrane was 

 found to envelope every joint on every side, and the longitudinal 

 fibres passed through it as in this figure. The longitudinal fibres 

 being probably the organs by which the motion of the column is 

 produced and regulated, and the membrane that by which fresh 

 matter, necessary for growth or for repairs of injury, is secreted.' 

 A decalcified section of a portion of the column, allowed to dry, 

 shows that the central canal is lined by the membrane and connected 

 to that between the plates, and thus with the longitudinal fibres. 

 (See Plate VI., Fig. 3.) 



^ See Buckland's Bridgewater Treatise, new edition, p. 393. 



2 A similar circle of minute pores is very perceptible round the central canal in an 

 oval plate of the column of a Platyerinus, from Mountain Limestone, in my collection. 



3 In Volume VI. of this Magazine, p. 351, some cases are described in which new 

 layers are formed round and inclose foreign matter attached to the column. 



