130 
FISHES OF THE CHALK. 
biting them. These casts, like tlie screw-stones 
of Derbyshire, must have been formed by the 
infiltration of siliceous matter, while in a fluid state, 
into the cavities of the shells, and which have been 
laid open and denuded by subsequent exposure 
to some agent capable of dissolving and removing 
the calcareous matter of the shell forming the 
matrix, while the siliceous casts remained un- 
altered.” 
Hippm'ites. — Specimens of the upper chamber 
of shells of this extraordinary genus, have lately 
been found in the chalk, near Lewes ; it is the first 
instance of their occurrence in Great Britain. I 
have named the species Hippurites Mortoni^ as a 
tribute of respect due to S. G. Morton, Esq. M.D. 
Secretary to the Academy of Natural Sciences of 
Philadelphia, whose important services to Geology 
I have already had occasion to notice. 
ICHTHYOLITES, OR THE FOSSIL REMAINS OF 
FISHES. 
The remains of fishes are of less frequent oc- 
currence in a fossil state than those of some other 
animals ; nor will this circumstance appear extra- 
ordinary, when it is considered that the softness 
of their structure, renders them liable to undergo 
putrefaction with great rapidity ; and that such as 
die a natural death, rise to the surface of the water, 
and immediately become the prey of a multitude 
of assailants. A concurrence of circumstances. 
