THE SILURIAN BEACH. 
57 
mother, but also in their emhryological develop- 
ment, which has many features in common with 
that of birds and turtles. Of this group, also, 
we find numerous remains in the ancient geologi- 
cal deposits ; and though we have not the means 
of distinguishing the species, we have ample evi- 
dence for determining the type. 
This combination of higher with lower fea- 
tures in the earlier organic forms is very striking, 
and becomes still more significant when we find 
that many of the later types recall the more an- 
cient ones. I have called these more compre- 
hensive groups of former times, combining char- 
acters of different classes, synthetic or prophetic 
types ; and we might as fitly give the name of 
retrospective types to many of the later groups, 
for they recall the past, as the former anticipate 
the future. And it is not only among the Fishes 
and the Reptiles that we find these combinations. 
The most numerous of the ancient Radiates are 
the Acalephian Corals, combining, in the Hydroid 
form, the Polyp-like mode of life, habits, and gen- 
eral appearance with the structure of Acaleplis. 
The Crinoids, with the closed cups in some, and 
the open, star-like crowns in others, unite fear 
tures of the present Star-Fishes and Sea-Urchins, 
and, by their stem attaching them to the ground, 
include also a Polyp-like character; while the 
Trilobites, with their uniform rings and their 
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