We have seen that the whole mass of the chalk is made up mainly of the remains of living things,--mostly of the microscopic foraminifera. We have seen that sponges were very plentiful in that ancient sea. Of other fossils we find brachiopods--different species of Terebratula and Rhynchonella--a large bivalve _Inoceramus_ sometimes very common; the very beautiful bivalve, _Spondylus spinosus_, belemnites, serpulae; and different species of sea-urchin are very common. A pretty heart-shaped one, _Micraster cor-anguinum_, marks a zone of the higher chalk, which runs along the top of our northern downs. Other common sea urchins are various species of _Cidaris_, of a form like a turban (Gk. _cidaris_, a Persian head-dress); _Cyphosoma_, another circular form; the oval _Echinocorys scutatus_, which, with varieties of the same and allied species, abounds in the Upper Chalk, and the more conical _Conulus conicus_. The topmost zone, that of _B. Macronata_, would yield a record of exuberant life, were the chalk soft and horizontal. There was a rich development of echinoderms (sea urchins and star fishes), but nothing is perfect, owing to the hardness of the rock (Dr. Rowe). The general difference in the life of the Chalk period is the great development of Ammonites and other Cephalopods in the Lower Chalk, and of sea urchins and other echinoderms in the Upper, while the Middle Chalk is wanting in the one and the other. Shark's teeth tell of the larger inhabitants of the ocean that flowed above the chalky bottom. Many quarries have been opened on the flanks of the Chalk Downs, of which a large number are now disused. They occur just where they are needed for chalk to lay on the land, the pure chalk on the north of the Downs to break up the heavy Tertiary clays, which largely cover the north of the Island; the more clayey beds of the Grey Chalk on the south of the downs to stiffen the light loams of the Greensand.[10] [Footnote 8: See _Common Stones_, by Grenville A. J. Cole, F.R.S. 1921.] [Footnote 9: 1,472 ft. at the western end of the Island, 1,213 ft. at the eastern.--Dr. Rowe's measurements.] [Footnote 10: Dr. A. W. Rowe.] Chapter VIII THE TERTIARY ERA: THE EOCENE