194 INTESTINAL STRUCTURE OF ICHTHYOSAURUS. 
possible extent of surface in the smallest space. 
A similar contrivance is shown by the Coprolites 
to have existed in the Ichthyosaurus. See PL 
15, Figs. 3, 4, 6* 
Impressions of the Mucous Membrane on 
Coprolites. 
Besides the spiral structure and consequent 
shortness of the small intestine, we have addi- 
tional evidence to show even the form of the 
minute vessels and folds of the mucous mem- 
* These cone-shaped bodies are made up of a flat and con- 
tinuous plate of digested bone, coiled round itself whilst it was 
yet in a plastic state. The form is nearly that which would be 
assumed by a piece of riband, forced continually forward into 
a cylindrical tube, through a long aperture in its side. In this 
case, the riband moving onwards, would form a succession of 
involuted cones, coiling one round the other, and after a certain 
number of turns within the cylinder, (the apex moving continu- 
ally downwards,) these cones would emerge from the end of the 
tube in a form resembling that of the Coprolites, PI. 15, Figs. 
3, 5, 7, 10, 11, 12, 13, 14. In the same manner, a lamina of 
coprolitic matter would be coiled up spirally into a series of 
successive cones, in the act of passing from a small spiral vessel 
into the adjacent large intestine. Coprolites thus formed fell into 
soft mud, whilst it was accumulating at the bottom of the sea, 
and together with this mud, (which has subsequently been indu- 
rated into shale and stone,) they have undergone so complete a 
process of petrifaction, that in hardness, and beauty of the polish 
of which they are susceptible, they rival the qualities of orna- 
mental marble. 
Fig. 6, shows a longitudinal section through the axis of 
