CHAPTER IX. 
FOSSIL ECHINIDA, OR SEA-URCHINS. 
Lien. 100. TURBAN ECHINUS, WITH ITS SPINFS; 3 vat. 
(Hemicidaris crenularis, Agassiz.) 
Jura limestone. 
Tue fossils we have now to examine are among the most 
familiar of the objects commonly known as petrifactions ; 
for as the enveloping cases of the Echini possess consider- 
able durability, they have served as moulds into which silex, 
cale-spar, limestone, pyrites, and other mineral substances, 
when in solution, or in a semi-fluid state, have percolated, 
and formed sharp and enduring casts, which exhibit the 
forms of the plates, and the disposition of the pores, striz, 
&e. of the original structures. 
