264 John Rofe — Notes on the Crinoidea. 



Limestone beds, particularly in the slabs of tbe Upper Silurian beds 

 at Dudley ; and the Apiocrinite bed of the Great Oolite covered by 

 Bradford Clay, described by Lyell,' is a very apt illustration of this 

 process. 



On the other hand, to account for the absence of these heads, it 

 has been suggested that the Crinoids may have been the larval state 

 of PalcBOComatula, as the recent Pentacriniis PJuropceus is of tlie 

 Comatula, and that the columns were the residue left when the 

 animal became free. But there is the serious objection to this theory 

 that no such free Comatula has been found fossil in any formation in 

 which the columns are abundant ; and although this is only negative 

 evidence, it acquires considerable weight when we find that the con- 

 struction of the column is opposed to this supposition, for they were 

 evidently of continuous, and probably of slow growth, increasing in 

 diameter by exogenous accretion, which may readily be elicited from 

 the columns themselves. At page 351 of the sixth volume of this 

 Magazine, this mode of the increase of the diameter of the column 

 was suggested from the fact that corals attached to them whilst 

 young and of small size became surrounded and invested by the new 

 layers so added. The same mode of growth may be inferred from 

 the side-arms, as the central canal of these branches off from that of 

 the column whilst the columns are small ; and as the diameter of 

 this increases simultaneously with that of the side-arm, a cup is 

 necessarily formed, increasing in depth as the column increases in 

 diameter, so that the side-arm in a full-grown specimen appears to 

 be inserted in a conical hollow or pit in the column (Fig. 1). 



A periodic enlargement of the diameter of the column is sometimes 

 indicated by the striae on the faces of the joints, as in well-preserved 

 specimens of some species these strias bifurcate at certain distances 

 as they recede from the centre (Fig. 3); and in weathered specimens, 

 as well as in slices cut for the microscope, where metamorphic action 

 has not effaced the structure, concentric rings of increase may fre- 

 quently be traced, similar in appearance to those observable in the 

 trunks of exogenous trees (Fig. 4). 



When these animals grew under modified circumstances, as in a 

 shallower sea, or in one disturbed by currents by which foreign 

 matter might be mixed with thein, but not in sufficient quantity to 

 destroy them, the texture of the rock produced would necessarily be 

 less uniform, and an inferior stone the result; so that, as a general 

 rule, the deeper the water in which the Crinoids lived, or the fui;ther 

 from land, the purer and harder would be the limestone deposit, 

 whilst the reverse would he the case in shallow water or near land. 

 By elevation of the coast and of the bed of the sea, a Crinoidal deposit 

 or colony might be brought nearer to land, the denudation frpm which 

 would form shale or sand-beds, as the case might be ; if these again, 

 by the oscillation of the sea-bed, should sink, further Crinoid deposits 

 might take place : and a repetition of this process may have been 

 going on for ages, as these deposits are found of great thickness, with 

 occasional beds of shale or other partings, sometimes thin, and at 

 1 Lyell's Student's Elements of Geology, 1871, p. 320. 



