178 HISTORICAL PALZONTOLOGY. 
peculiar in consisting each of three, four, five, or more rows of 
large imperforate plates, whilst there are sometimes four or ten 
rows of plates in the “ambulacral areas” also: so that there 
are many more than twenty rows of plates in the entire shell. 
Some of the Palzeozoic Sea-urchins, also, exhibit a very pecu- 
liar singularity of structure which is only known to exist in a 
very few recently-discovered modern forms (viz., Calveria and 
Phormosoma). The plates of the inter-ambulacral areas, 
namely, overlap one another in an imbricating manner, so as 
to communicate a certain amount of flexibility to the shell ; 
whereas in the ordinary living forms these plates are firmly 
articulated together by their edges, and the shell forms a rigid 
immovable box. The Carboniferous Sea-urchins which ex- 
hibit this extraordinary peculiarity belong to the genera Lefv- 
dechinus and Lepidesthes, and it seems tolerably certain that 
a similar flexibility of the shell existed to a less degree in 
the much more abundant genus Archeocidaris. ‘The Carbon- 
iferous Sea-urchins, like the modern ones, possessed movable 
spines of greater or less length, articulated to the exterior of 
the shell; and these structures are of very common occur- 
rence in a detached condition. The most abundant genera 
are Archeocidaris and Palechinus; but the characteristic 
American forms belong principally to JZelonites, Oligoporus, 
and Lepidechinus. 
Amongst the Azme/ides it is only necessary to notice the little 
spiral tubes of Spzvorbis Carbonarius (fig. 120), which are 
Fig. 120.—Spirorbis (Microconchus) Carbonarius, of the natural size, attached to a fossil 
plant, and magnified. Carboniferous, Britain and North America. (After Dawson.) 
commonly found attached to the leaves or stems of the Coal- 
plants. ‘This fact shows that though the modern species of 
Spirorbis are inhabitants of the sea, these old representatives 
of the genus must have been capable of living in the brackish 
waters of lagoons and estuaries. 
The Crustaceans of the Carboniferous rocks are numerous, 
