314 THE MEDALS OF CREATION. Cuap. IX. 
modern authors; but I found the attempt hopeless, from 
the changes in nomenclature and arrangement which are 
constantly taking place. The monographs on the fossil 
Kchinidee, by Prof. E. Forbes, now in course of publication 
by the Government School of Mines, will, when completed, 
place this branch of Paleontology on a satisfactory basis. 
For the convenience of study, the fossil Echini may be 
separated into three principal groups ; viz. the CIDARITIDA, 
or Turban Echinites ; the CLYPEAsTRIDz, or Buckler-shaped 
KEchinites ; and the Sparancip&, or Heart-shaped Echinites. 
Crparitipz. Turban Echinites.—In these sea-urchins 
the vent is situated on the summit of the shell, and is sur- 
rounded by five minute apertures for the exclusion of the 
ova. The mouth, or oral aperture, is placed directly oppo- 
site, in the middle of the base, or inferior surface. The 
mouth is large, and furnished with a powerful apparatus, 
armed with teeth; which is sometimes found fossil (see 
Jig. 1, 3, Lign. 101). The structure of the animal, in conse- 
quence of the symmetrical position of the two chief outlets 
of the shells, is strictly radiated ; that is, all the parts proceed 
from, or are arranged around one common centre. The 
tubercles are larger and fewer than in the other tribes of this 
family ; they support long and powerful spines, which vary 
much in form and ornament in different species (see Lign. 
102). The larger tubercles and spines are beset with smaller 
ones, disposed in regular series. 
The Turban Kchinites are the most ancient types of the 
order, some forms appearing in the Carboniferous deposit. 
The species are very numerous. The echinites of this group 
are subdivided into four tribes : 
1. Cidares, properly so called.—In these the tubercles are 
perforated ; the ambulacra narrow, and beset with granular 
tubercles, and the two lines of pores are close together. 
The pseudopodia can be protruded to a great length, even 
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