OOPROLITES. 193 
Spiral Disposition of Small Intestines. 
As the more solid parts of animals alone, are 
usually susceptible of petrifaction, we cannot 
demonstrate by direct evidence the form and size 
of the small intestines of the Ichthyosauri, but 
the contents of these viscera are preserved in 
such perfection in a fossil state, as to afford 
circumstantial evidence that the bowels in which 
they were moulded, were formed in a manner 
resembling the spiral intestines of some of the 
swiftest and most voracious of our modern 
fishes. 
We shall best understand the structure of 
these intestines by examining the corresponding 
organs of Sharks and Dog-fish, animals not 
less peculiarly rapacious among the inhabitants 
of our modern seas, than the Ichthyosauri were 
in those early periods to which our considera- 
tions are carried back. We find in the intestines 
of these fishes, (see PI. 15, Figs. 1, and 2,) and 
also in those of Rays, an arrangement resembling 
that of the interior of an Archimedes screw, 
admirably adapted to increase the extent of 
internal surface for the absorption of nutriment 
from the food, daring its passage through a tube 
containing within it a continuous spiral fold, 
coiled in such a manner, as to afford the greatest 
GEOL. O 
