Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/156

 152 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 similar contrivance is shown by the Coprolites to have existed in the Ichthyosaurus. See Pl. 15, Figs. 3, 4, 6. 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 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 continually downwards,) these cones would emerge from the end of the tube in a form resembling that of the Coprolites, Pl. 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 indurated 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 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 another 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 the 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 the large intestine, would coil up spirally within it, until it attained the largest size admitted by its diameter; from this coil successive portions would be broken off abruptly, (at b,) and descending into the cloaca would be thence discharged into the sea.

Besides the spiral structure and consequent shortness of