11 
Clay, and that of Sheepy, and several other places, art 
considered as detached portions of the highest known 
stratum but one, which is Sand. As it lays not far above 
the Chalk, it may be sought for in those districts which are 
bounded by Chalk, but as Mr. Farey has observed to me, 
in a valuable letter upon this subject, this u being the 
uppermost stratum but one, it is mostly denudated and 
gone; and, except in some particular tracts in and near 
where the Sand upon it is found, this blue High gate 
Clay will be found only in particular hummocks or isolated 
patches on the Clays and Sands beneath it (in which the 
London wells are sunk).” Mr. Farey has also favoured 
me with a detail of the boundaries of the three great tracts 
on which the strata covering the Chalk are found; the 
Northernmost extending along the coast a considerable way 
on each side of the mouth of the Humber; the middle 
or greater tract covering the South of Norfolk, greater 
part of Suffolk, Essex, Middlesex, parts of Berkshire, and 
Wiltshire, the South of Hampshire, Surrey, and Kent; 
the Southern tract extends from Brighton to Axminster in 
Devonshire. It would be doing a service to Geology, if 
persons resident in these tracts would search for and com- 
pare the fossils with each other. 
NAUTILUS centralis . — Left hand figure . 
Spec. Char. Involute, umbilicate ; aperture bluntly 
lunate, septa? entire, concave, not recurved at 
their ends; siphunculus central. 
This Shell is about three-fourths of its diameter in thickness, 
and the concavity of the septae is gentle and regular without 
recurving; see the outline below it. The distances of the 
