Mollusca found in the Chalk, 80 (of which 19 make their first appearance at that period) are living at the present day. At the same time it is not in the deep Atlantic, but rather on its warmer coasts, that we now find the widest dispersion of this group of cretaceous genera.
These constitute the main points of resemblance. Striking as they are, their limits are confined ; and, on the other hand, the divergences are great. The great feature of the Chalk-fauna is the abundance of Cephalopods and its large reptiles, so very few genera of which have descended to present times. The relative distribution of genera of the principal Invertebrata was, according to D'Orbigny *, before the late discoveries, approximately as under : —
Number of genera.
In the Extinct White Chalk. with the Chalk. Living.
Cephalopods 13 12 1
Gasteropods 41 9 32
Lamellibranchs 52 11 41
Brachiopods 10 3 7
Echinoderms 34 27 7
150 62 88
With the Cretaceous series disappear the many genera of Cephalopods allied to Ammonites which range through the Triassic and Jurassic formations, together with the Rudistes, so peculiar to the
Chalk period.
The Mollusca of the deep Atlantic are still undergoing investigation at the hands of Mr. Gwyn Jeffreys. Enough, however, is already known of the remarkable results to form some opinion of its great value in a geological point of view. The total number now dredged may probably exceed 300 species. Of these, 86 species have been recognized and are noticed in the Reports now published. A very large number are altogether new. With the aid of Mr. Jeffreys, I have drawn up the following lists, which will serve to show the light which may be thrown on the palaeontology of our more recent Tertiary strata by these inquiries. Of the 86 determined species, 67 were known before as living, though chiefly as high Northern and Arctic species (while the wider range of some few of the species is very remarkable), and 19 are fossil species
- Paleontologie et Geologie stratigraphiques, vol. iii. § Terrain Senonien.
