the Secondary age came to an end, the fullness of the Ammonite generations disappeared for ever. In the succeeding Tertiary period not a single Ammonite of any kind occurs; the genus Nautilus remaining in the Tertiary period—as it survived into the Mesozoic or middle period—as the sole representative of a once plentiful four-gilled population.
If the history of the four-gilled cuttle-fishes is thus plainly told as having its beginnings in the Palæozoic period, its maximum development in the Mesozoic period, and its lingering presence in the Tertiary period, the two-gilled cuttle-fishes may be said to possess an equally interesting history. Compared with their four-gilled neighbors, the two-gilled forms are late comers upon creation's scene. Not a single fossil two-gilled form occurs in all the Palæozoic period, extending from the Laurentian to the Permian rocks. If they existed in Palæozoic seas, they have at least left no trace of their presence. Their softness of body may perchance have contributed to their elimination from the oldest fossil records; but, laying aside mere conjecture, we find the first fact of the past history of the two-gilled forms in the presence of the fossil shells of the extinct Belemnites in the Triassic rocks. The Belemnites themselves disappear at the close of the Mesozoic period; but fossilized shells of species allied to our living Sepias occur in the Oolite; and the internal shells of squids are found in the Lias or lower Oolites. In the Tertiary rocks, Argonaut (Fig. 4) shells occur in the Pliocene deposits; the Eocene rocks also give us sepia remains; and various other two-gilled fossils (Beloptera, etc.) are found in Eocene and Miocene formations.
Briefly summarized, then, we find that the chief details in the past history of the cuttle-fishes are told when we are reminded that the four-gilled forms are by far the more ancient of the two groups; that they first appear in the Silurian rocks, while the two-gilled forms appear first in the Secondary rocks; and, lastly, that the record of the one group is the converse of the other. For, the four-gilled species attained their maximum in the Primary and Secondary rocks, and have practically died out, leaving the pearly nautilus as their sole representative in existing seas. The two-gilled race, starting in the Secondary rocks, and leaving the extinct belemnites as a legacy to the past, have, on the other hand, flourished and progressed, and attain their maximum, both in size and numbers, in the existing seas and oceans of our globe.
What ideas concerning the origin and evolution of these animals may be legitimately deduced from the foregoing facts of their structure and distribution in time? In the answer to such a question, asked concerning any group of living beings, lies the culminating point of all biological science. That the cuttle-fishes fall nominally into their place in the scale of being indicated by evolution, and that in their individual development, in the growth of their special organs, such as