a 
CIDARITIDZ OF THE PALZOZOIC ROCKS. Sau 
_ the imprint indicate tubular cavities in the flint, formed 
__ by the spines, and show that these processes were attached 
_ to the shell when the latter was enveloped by the fluid silex ; 
_ the case and the spines having since perished. But in the 
Chalk, exquisite specimens of Cidarites occur with the case 
perfect, and filled with flint : examples of this kind are often 
attached to a nodule by the slender column of silex that 
fills up the aperture of the shell. The mineralized condi- 
tion of the originally friable calcareous cases of Cidarites 
_ and other Echini, is worthy of attention: for whether the 
shell or spines be imbedded in chalk, flint, or pyrites, if the 
structure and form remain, the constituent substance is 
invariably opaque crystallized carbonate of lime, having an 
oblique fracture. 
As this conversion of a crustaceous envelope into calc- 
spar is constant, it has probably resulted from the peculiar 
nature of the original animal structures; but the cause of 
~ such transmutation is unknown. 
CIDARITIDZ OF THE Patmozorc Rocxs.—Three genera of 
_ this family, comprising several species, have been discovered 
in the carboniferous limestone of Northumberland and Ire- 
land. One genus is undistinguishable from Cidaris; and 
_ the species are placed under that name in Mr. Morris’s Cat. 
Brit. Foss. ; 
These fossils have been figured and described by Prof. 
John Phillips, and Col. Portlock. Prof. M‘Coy, with his 
accustomed penetration and sagacity, has ascertained, that 
notwithstanding the general resemblance between the Cida- 
rites of the secondary and those of the paleozoic formations, 
_ the latter are constructed on an entirely different plan. In 
the turban echini of the secondary, tertiary, and modern 
Seas, the interambulacral plates always consist of two rows; 
but in the paleeozoic Cidarites there are three or some greater 
odd number of these plates. This eminent naturalist, there- 
VOL. I. ¥ 
