John Rofe — Notes on the Crinoidea. 263 



detadied and dispersed, in these interjacent shale-beds, the arms and 

 columns generally entirely removed from them, or with exceptionally 

 a very few plates left attached. The nature of the parting beds 

 varies very much. In some cases they appear to consist principally of 

 broken or small fossils, with a mixture of more or less argillaceous 

 matter; and these beds are generally thin and friable, and may be 

 called rotten, and in them many fossils are found which may easily 

 be freed from the matrix whilst fresh from the quarry. In other 

 cases the parting beds are sandstone, and in others argillaceous shale, 

 these being generally the thickest. 



These various conditions of the Limestone alluded to may arise 

 from differences of the circumstances, both as to the depth of the 

 sea under which the fossils were deposited, and the vicinity of land. 

 Let us consider first the hard rock or marble. This, from the 

 structure, appears to have been deposited in deep clear water, because 

 the calcareous base or cement which holds the fossils together is so 

 transparent and free from foreign matter that their organization may 

 frequently be distinctly seen when the marble is polished. But in 

 this marble, as above stated, very little besides portions of columns 

 is found, although, from the thickness of the beds, they must have 

 been a long time accumulating, and are evidently the product of 

 many succeeding generations. If this be so, the question at onco 

 arises — What has become of the heads ? In endeavouring to answer 

 this, I must refer to some notes on the Crinoidea published in Vol. VIII. 

 of this MAaAziNE, p. 241, in which it was suggested that all the 

 ossicles and plates of these animals were invested in and held 

 together by a membrane, which, in case of the death of the animal, 

 would decompose, and thus allow the plates to fall asunder. The 

 smaller surfaces in contact in the joints of the comparatively thin 

 plates of the head, and of the arms and pinnules, would be the first 

 so to decay, and the plates and ossicles would be dispersed before 

 the decomposition of the larger surfaces between the columnar plates 

 would permit their separation. These, however, would eventually 

 fall into a mass, and be cemented together in a manner similar to 

 the Coral Eeefs described by Mr. Jukes, ^ and thus form a bed for a 

 succeeding generation, which one after another would follow the 

 same course so long as the circumstances under which the growth 

 and deposit may take place remain the same ; and the great thickness 

 of some of the Crinoidal beds is sufficient evidence of long-continued 

 repetition, and the beds so deposited would probably be similar in 

 every respect as to the hardness and quality of the stone. But if 

 from any cause the sea became more shallow, the nature and hard- 

 ness of the bed would be different; and if currents brought the 

 surface soil denuded from a neighbouring land, and deposited it on 

 these animals, they would be destroyed hy it and buried before de- 

 composition took place, and they would thus be held together and 

 preserved nearly or perhaps entirely whole, and the denuded soil 

 would form the parting bed on the limestone, and thus cover and 

 protect the fossil. That this has been the case is evident in many 

 ^ Jukes and Geikie's Manual of Geology, 1872, pp. 388 and 389. 



