1 7 
would fill up the small spaces existing at the articulation of the plates ; 
and thus would be formed the bases, or margins, of these hexagonal 
conical cells. These being formed, he thinks, on the principle that 
homogeneous particles are most likely to unite, that the successive 
approximation of crystallising particles will take place on these hexa- 
gonal crystallized margins, rather than on the intervening spaces of 
the shell itself; and that, by the gradual approximation of the lines of 
crystals they formed, have resulted the conical cavities described. 
A specimen which I possess, being the internal part of an echinite 
from the Kentish chalk-pits, will serve very much to illustrate and 
confirm the observations of M. Walch. The crystallizations of cal- 
careous spar are here seen formed on the internal surface of the 
plates, the basis of the crystals being the margins of the plates. In 
the silicious nucleus of M. Walch, the crystals had formed an hexa- 
gonal cavity ; but, in this calcareous mass, the crystals are solid : a 
difference which might proceed from the silicious crystals, in the 
former case having been formed on the calcareous crystals, which, 
being afterwards removed, would necessarily leave the inferior part 
of the silicious crystals hollow. 
Small specimens of cidares, in a pyritous state, are sometimes 
tound, with other fossils, in the Isle of Portland. Very minute shells 
o 1 11s ind are also found in the Devonshire whetstone, in the state 
o calcedony : they are also found in a silicious state in the green 
sand of Wiltshire. 
The echini of the second section, or division, of anocysti, are dis- 
mguished as Chjpei, from their similitude in form to the round 
bucklers of the foot-soldiers of the ancients. The first species of 
these is, Clypeus sinuatus, Lesk. the Echinus sinuatus, Linn Plate 
II. Fig. i. The upper surface is convex, and divided into ten area! 
by ten striated ambulacra. One of the area is also divided by a 
groove, hollowed out from the centre of the shell to the margin. 
Ihe ambulacra, at parting from the centre of the shell, expand, but 
D 
