THE DEEP-SEA FAUNA. 155 
On comparing the marine fauna of some of the older forma- 
tions with that of recent ones, we are struck with the similarity 
of the types. In the silurian we find crinoids, echini, starfishes, 
crustacea, mollusca, and fishes, an association of types like those 
of our epoch. Some groups, however, attained at that early 
day a preponderance such as none of the types of the present 
epoch have ever reached. 
There is nothing in our living fauna, for instance, analogous 
to the immense development of the erinoids or of the trilobites, 
the most prominent crustacea of the silurian period, which dis-. 
appeared abruptly at the time of the coal period, and are rep- 
resented to-day only by our horseshoe crabs. The ammonites 
— which, like their distant allies of to-day, the nautilus, argo- 
naut, and spirula, must have swept over the surface of the seas, 
or sunk to its greatest depths — have had a most remarkable 
history. From straight gigantic shells, nearly rivalling in size 
the huge squid of to-day, they have passed through a series 
of transformations, including types of the most complicated 
combinations, and finally seem to have succumbed to the ex- 
quisite perfection of their type, and to have been killed in the 
struggle for life. They were slowly relegated to interior seas, 
and, ceasing to advance, gradually died out. 
The depths of the seas seem at first glance the safest of all 
retreats, — the secret abysses where the survivors of former geo- 
logical periods would be sure to be found. Yet oceanic dredg- 
ings have not brought to light as many of the ancient types as 
the more enthusiastic dredgers had led us to expect. They have, 
however, given us a large number of animals living in deep 
water, where they have been subjected to no violent changes, to 
which no revolutions of the surface of the earth can extend, and 
where the only changes are probably those of temperature, — 
animals living now in the depths of the sea, under much the 
same conditions as those which prevailed during the last days of 
the jurassic period. 
The conclusion drawn from these facts by Lovén, Moseley, 
Perrier, and others is that the abyssal fauna has descended 
from the littoral and other shallow regions, to be acclimatized 
at great depths. The conditions of existence becoming more 
