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 Dysaster, 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 Cidaridee, Echinothuride, 
and irregular urchins, and the disproportionate num- 
bers of the genera Astropecten, Astrogonium, 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 Terebratulina 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 
