152 INTESTINAL STRUCTURE OF ICHTHYOSAURUS. 



surface for the absorption of nutriment from the food, during 

 its passage through a tube containing within it a continuous 

 spiral fold, coiled in such a manner, as to afford the greatest 

 possible extent of surface in the smallest space. A simi- 

 lar 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 



* These cone-shaped bodies are made up of a flat and continuous 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 conti- 

 nually 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, colling one round the other, and after a certain numberof turns within 

 the cylinder, (the apex moving continually 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, l^. 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 indurated into shale and stone,) they have undergone so complete a 

 process of petrifi^ctlon, that in hardness, and beauty of the polish of which 

 they are susceptible, they rival the qualities of ornamental marble. 



Fig. 6, shows a longitudinal section through the axis of a coprolite, from 

 the inferior chalk, in which this involute conical form is well defined. Fig. 

 4, is the transverse section of anotlicr Coprolite from the lias, showing the 

 manner in which the plate coils round itself, till it terminates externally in a 

 broken edge (at b.) In all the figures tlie letter b, marks the transverse 

 section of this plate, where it is broken off near the termination of its outer 

 coil; the sections at b, show also the size and form of the flattened passage 

 through the interior of the screw. 



A lamina of tenacious plastic substance pressed continually forwards from 

 the interior of such a screw, into the cavity of tlie large intestine, would 

 coil up spirally within it, until it attained the largest size admitted by its ■ 

 diameter; from this coll successive portions would be broken off" abruptly, 

 (at b,) and descending into the cloaca would be thence discharged into the 

 sea. 



