36 GEOLOGICAL SURVEY 
group Echinoderma. These have, for the most part, either an external shell, composed of alter- 
nating rows of perforated and imperforated plates, fitted together so as to form a globular covering, 
or a calcareous skeleton, covered with a rough integument. 
Prof. Agassiz, who studied this group with his usual skill, has divided it into five families, as 
follows. 
1. Spatangoidea, which have the shell oblong, the mouth placed anteriorly, and the anus or vent 
at the posterior end. 
2. Clypeastroidea—shell oblong or circular; mouth near the centre, open; vent posterior. 
3. Cidarides—shell globular; mouth central, below; vent in the centre of the summit. 
4. Asterides—are more or less star-shaped—have the mouth and anus central, below; rows of 
pores or ambulacre on the under side of the rays. 
5. Crinoidea—animal sessile; supporting column, round, oval or angular, articulated ; cup con- 
taining the viscera composed of plates; arms proceeding from the upper edge of the cup, divided 
into tentaculated fingers, which surround the mouth. 
These families are again divided into numerous genera ; but as Prof. Agassiz is about to revise 
the Echinoderma from all our formations, it will be better to present nothing further on the subject 
at this time. 
Class Polypi—tn this class we find the Anthozoa, or flower animals—the fabricators of corals. 
It would be difficult to estimate the influence that these silent little workers have exerted upon the 
rocky strata of our globe, during the vast period since their first appearance in the early seas. 
Even in the present seas changes are produced that are sufficient to show us that that influence 
must have been very great. For besides the calcareous frame work of the animals themselves, 
which often makes up entire strata of rocks, there is a white calcareous mud deposited in the 
vicinity of coral reefs that must, of itself, form very thick beds. On the coast of Florida such beds 
are at this time accumulating, and are composed of the mud just mentioned, and of comminuated 
corals. The islands, of which corals form the foundation, and the harbors obstructed by these 
little architects, are familiar knowledge. 
If there be no other modifying circumstance than that of temperature to affect the existence of 
these animals, their remains, embedded in the rocks of former periods, afford us the means of 
estimating the temperature of the seas in which they lived. The temperature required for the 
existence of the coral-building polypi, as ascertained by the Exploring Expedition, is about 75°, and 
hence we have but few corals north of Florida. Another curious fact in their economy is that they 
do not live at great depths in the ocean, but within a few fathoms of the surface. Nevertheless, 
soundings off coral reefs show that they extend to a great depth—a fact that cannot be well 
accounted for but by supposing a gentle subsidence of the reef to take place, at the same time that 
the coral insects continue their operations above, and the mass increases at about the same rate at 
which the subsidence takes place. 
Class Infusoria—The exquisite forms of the covering of the astonishingly minute fossil shells 
that belong to animals of this class, are abundant, and consequently pretty well known. ‘They are 
often found forming pretty thick beds at the bottom of ponds and pools of stagnant water, but the 
most remarkable deposits occur at Richmond and Petersburg, Va. At the latter place the bed is at 
least forty feet in thickness. It isa marine deposit, and encloses numerous fossil shells of the 
a 
