490 THE DEPTHS OF THE SEA. [chap. x. 



fossils, so that their presence apparently in abun- 

 dance in the recent chalk-mud is a clear instance 

 of the preservation of one of the old types hitherto 

 supposed to be extinct. The same may be said of 

 Pourtalesia, which must associate itself either with 

 Ananchytes or with Di/saster, both of which are 

 types of groups likewise supposed to have been lost. 

 We thus find that, while no Echinoderm hitherto 

 discovered in the deep water is specifically identical 

 with any chalk form, not only does the abyssal fauna 

 with its abundance of the Cidaridae, Echinothuridae, 

 and irregular urchins, and the disproportionate num- 

 bers of the genera Astropecten, Astrogonhim, and 

 Stellaster, and their allies among starfishes, singu- 

 larly resemble the chalk in general facies ; but 

 several genera approach chalk forms more closely 

 than they do any hitherto known in a living state 

 — approach them so closely as almost to force upon 

 us the conviction that their relation is one of descent, 

 accompanied by change of conditions and consequent 

 modification, though not to any extreme degree. 



As I have already stated, the whole of the mol- 

 lusca from the deep water which had been previously 

 described as fossils were known from tertiary and 

 post-tertiary beds ; with the very doubtful exception 

 of our common TerebratuUna caput-serpentis, which 

 certainly approaches very closely Terebratula striata 

 from the chalk. 



It is not surprising that this should be the case. 

 It is a marked character of the European Tertiaries 

 that with the exception of some of the older beds in 

 the south of Europe, all of them have been deposited 

 in shallow water ; so that the tertiary beds represent 



