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But every device if we may so regard these changes is useless, and the devastating agency of the Cretaceous, whatever it was, removes the Ammonites and Belemnites from the scene. The Mollusc world, like the world of plants and of reptiles, approaches its modern aspect. In the fish world, too, there is an effective selection in the course of the Cretaceous.
These coralline strata extend through the calcareous hills of the north-west of Berkshire, and north of Wilts, and again recur in Yorkshire, near Scarborough. Nerinaea Goodhallii, Fitton. Belemnites hastatus. Ammonites Jason, Reinecke. A. Elizabethae, Pratt. Belemnites Puzosianus, d'Orbigny. B. Owenii, Pierce. Oxford Clay, Christian Malford, Wiltshire. a.
Thus the chalk contains remains of those strange flying and swimming reptiles, the pterodactyl, the ichthyosaurus, and the plesiosaurus, which are found in no later deposits, but abounded in preceding ages. The chambered shells called ammonites and belemnites, which are so characteristic of the period preceding the cretaceous, in like manner die with it.
These zoological results are curious and unexpected, since it was imagined that we might look in vain for the carnivorous trachelipods in rocks of such high antiquity as the Great Oolite, and it was a received doctrine that they did not begin to appear in considerable numbers till the Eocene period, when those two great families of cephalopoda, the ammonites and belemnites, and a great number of other representatives of the same class of chambered shells, had become extinct.
They abandoned the defensive system of warfare and boldly took up the offensive. No doubt, like their descendants, the belemnites were exceedingly active and voracious creatures. FISHES AND AMPHIBIANS. In the Triassic and Jurassic, little progress was made among the fishes, and the ganoid was still the leading type.
In passing from the Cretaceous to the Tertiary, we enter upon the threshold of our modern world. We leave behind all the great wonders of the old world, the gigantic reptiles, the forests of tree ferns, the seas full of ammonites and belemnites, and come among the no less wonderful but more familiar modern forms.
Only the largest and heaviest belemnites are known as thunder stones; the smaller ones are more commonly described as agate pencils. In Shakespeare's country their connection with thunder is well known, so that in all probability a belemnite is the original of the beautiful lines in 'Cymbeline': Fear no more the lightning flash, Nor the all-dreaded thunder stone,
Here, I knew, I could meet only dead men, but urged by some curiosity, I searched to the end, wading in the middle through a three-feet depth of sea-weed twine: but there was no one; and only belemnites and fossils in the chalk.
It partakes of the character both of the Cretaceous Belemnites and of the living Cuttle-Fish, and is known as the Spirulirostra. Another very characteristic group among the Tertiary Shells is that of the Nummulites, formerly placed by naturalists in immediate proximity with the Ammonites, on account of their internal partitions.
The animal carried an ink sac, and no doubt used it as that of the modern cuttlefish is used, to darken the water and make easy an escape from foes. Belemnites have sometimes been sketched with fossil sepia, or india ink, from their own ink sacs. In the belemnites and their descendants, the squids and cuttlefish, the cephalopods made the radical change from external to the internal shell.
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