CHAP. x.J CONTINUITY OF THE CHALK. 487 



from the coast of Portugal more nearly allied to 

 chalk forms than to any others, but it is in the 

 Echiriodermata that the peculiar relation between 

 the ancient and the modern faun'ce becomes most 

 apparent. To review briefly the chief points bearing 

 upon this question. The Apiocrinidso, the group of 

 fixed crinoids which I have already described, are 

 abundant throughout the whole range of the Jurassic 

 rocks, their remains being frequently very abundant 

 in the thick cream-coloured limestone beds of the 

 oolites. Towards the close of the Jurassic period, 

 the typical genera disappear, and in the chalk we 

 find the group represented by an evidently degenerate 

 form, Bourguetticrinus. In some tertiary -beds frag- 

 ments of the stems of a small Bourgnetticrinus have 

 been found, and such were likewise discovered in 

 the recent lime breccia of Guadaloupe, which con- 

 tained the well-known human skeleton now in the 

 British Museum. There can be little doubt that 

 these tertiary and post-tertiary fragments are to be 

 referred rather to the genus Rhizocrinus, which we 

 now know to be so widely distributed, living, in 

 deep water. Now in this series of Apiocrinidse, 

 extending from the Eorest marble to the present 

 time, although there is a succession of constantly 

 changing species, yet the gradual degradation in 

 development in the same direction throughout the 

 series seems to point unmistakeably to some form 

 of continuity, to a type gradually succumbing to con- 

 ditions slowly altering in an unfavourable direction. 



The other family of the stalked crinoids, the 

 Pentacrinidse, are in a different position. They are 

 abundant in the Lias ; very abundant in the lower 



