540 SUPPLEMENT 
zoologists of his school, comprehended all the animals, more 
or less orbicular, whose cretaceous envelope bristles with a 
number of spines, of a form more or less variable, and con- 
stantly calcareous. This has caused these animals to be 
compared to hedge-hogs, and accordingly, they are popularly 
termed sea hedge-hogs, or urchins, and sometimes sea chest- 
nuts. But at the present day, among modern zoologists, since 
the researches of M. de Lamarck, this name is reserved for a 
certain number of species, for those which really better merit 
the title, in consequence of the long spines with which they 
are armed. . 
The exterior envelope, which determines the form of an — 
echinus, (which is described in the text) can be compared to 
nothing which exists in other animals. In the greatest part 
of its extent, it is formed by two membranes, one external, 
and thicker, the other internal, and so thin, that the name of 
pellicle perfectly suits it, and between which there is a toler- 
ably thick and solid testa, completely calcareous, and com- 
posed of a great number of small polygonal pieces, evidently 
immovable, but not cemented, at least, during the growth of 
the animal. In the neighbourhood of the mouth and of the 
anus, the skin is not thus solidified, accordingly, it is sensibly 
thicker, and much more resistant. 
The testa of the echini is entirely calcareous, almost without 
any mucilaginous or animal part, and is fibrous perpendicu- 
larly at the surfaces, which proves that the mode of growth, 
though taking place upon the edges, nevertheless differs much 
from the same operation in the shell of the mollusca. 
The pieces which constitute the testa of an echinus may 
be divided into coronal, and terminal. ‘The coronal are those 
which by their union form the most important part, the most 
extended, and which circumscribe the body in its circumfer- 
ence; and the terminal are those which surround the buccal 
orifice, and the anal orifice, and which fill the two apertures 
