PRINCIPAL FORMS OF SEA-URCHINS. 205 
Taking a rapid survey of the principal forms of sea- 
urchins, we may divide the class of Hchinoidea into two or- 
ders: the Palechinida, or older sea-urchins, in which the 
shell is composed of more than twenty rows of plates; and 
the Autechinida with twenty rows of plates.* 
Order 1. Palechinida.—Comprises first the suborder Me- 
lonitida, in which there are more than ten rows of ambula- 
cral plates, represented by Melonites of the coal formation, 
and Protechinus, Palechinus, Archeocidaris, etc. In the 
second suborder HLocidaria, there are ten rows of ambulacral 
plates. A type of the group, Locidaris Kaiserlingtt, appears 
in the Permian formation. 
Order 2. <Autechinida.— 
To this division belong sea- 
urchins with twenty rows of 
plates. The first suborder is 
the Desmosticha, comprising 
those sea-urchins with band- 
like ambulacra extending 
from the mouth to the oppo- 
site extremity, and of more 
or less regular, flattened, 
spherical form. Such are 
Cidaris, Echinus, Echinom- 
‘ Fig 149 —Hchinarachnius parma, com- 
etra, Clypea ster, and chi- monSand-cake. Natural size.—After A, 
A 3 Agassiz. 
narecshnius. The Fchinus 
esculentus Linn., of the Mediterranean Sea, is as large as 
an infant’s head, and is used as an article of food. 
In Clypeaster the body is large and the shell very solid. 
C. subdepressus Agassiz is common on the Floridan coast. 
An orbicular flattened type are the sand-cakes, of which the 
Echinarachnius parma Gray (Fig. 149) is abundant in the 
shallower portions of the North Atlantic, from low-water 
mark to forty fathoms. It is replaced southward from 
Nantucket to Brazil by Mellita testudinata Klein. 
The last suborder, Petalosticha, is characterized by the 
* These are terms proposed by Haeckel, who regards these divisions 
as subclasses, but we think they should more properly be called orders. 
