39 
as they do under the influence of the sun and air, 
would hardly be expected to leave their traces im- 
pressed on ancient rocks. Professor D’Orbigny, how- 
ever, has watched the dissolution of these animals on 
the sea-shore, and found that, after wasting, they may 
leave their impressions on the sand; which, not being 
disturbed by a high tide for nearly a month, retains 
the impression of the zoophyte, and serves as a mould 
to receive materials which take a cast and transmit it 
to subsequent ages. We find one of these impressions 
on the slab of the Anomcepus Scambus; and President 
Hitchcock, having examined it, is of opinion that it 
retains the traces of a Medusa. ‘The impression is 
about five inches in diameter, of a darker color and 
smoother texture than the rest of the rock. Its edges 
fade away gradually in the surface of the subjacent 
sandstone. A similar impression is found on the 
superior surface of the slab containing the Argozoum. 
GROUP TENTH. 
The tenth group contains the Harpacopvus, a name 
derived from épzay7, seizure, rapine. It is represented 
by President Hitchcock as having the form of a drag. 
The figure given by him resembles in a degree the 
