17 



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 con- 

 firm the observations of M. Walch. The crystallizations of calcareous 

 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. VValch, the crystals had formed an hexagonal cavity ; but, in this 

 calcareous mass, the crystals are solid : a difference which might pro- 

 ceed 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 found, 

 with other fossils, in the Isle of Portland. Very minute shells of this 

 kind are also found in the Devonshire whetstone, in the state of cal- 

 cedony : they are also found in a silicious state in the green sand 

 of Wiltshire. 



The echini of the second section, or division, of anocycysti, are distin- 

 guished as Clypei, 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 areae by ten striated ambulacra. 

 One of the areae is also divided by a groove, hollowed out from the cen- 

 tre of the shell to the margin. The ambulacra, at parting frotn the 

 centre of the shell, expand, but contract at the margin, and thus con- 

 tinue to where they meet in the centre of the lower part of the shell, 

 which is rather excavated and grooved where the ambulacra pass. The 

 whole of the surface c-f the shell is thickly beset with granular tuber- 

 cula, the largest of which are surrounded by small circular risings. This 

 species is figured by Plott, Tab. u. 9, 10, and is found chiefly at Tang- 

 ley, Fulbrook, and Burford, in Oxfordshire : they are also found in 



VOL. III. D 



