The ammonids, or “ammonites,” were extremely abundant throughout the world during Mesozoic time. Their shells were chambered like those of the pearly nautilus, a related cephalopod mollusk inhabiting tropical seas at the present time. While only four species of the Nautilus tribe are living today, thousands of species of ammonids swarmed the prehistoric seas. Many new forms came into existence in Triassic time but few survived the period. A pronounced revival occurred in the Jurassic, only to be followed by a decline and eventual extinction at the close of the Cretaceous. Ammonites measuring three or four inches across the diameter of the coiled shell were about average size, but diameters up to three or four feet were not uncommon. Externally the shells were ornamented with ribs, knobs, and spines; inside was a pearly lining. The partitions were thin and composed of the same pearl-like substance as the lining. Each partition becomes wavy as it approaches the shell, and the line of union has a distinct pattern which is seen in specimens which have lost the outer shell layer. This wavy suture line becomes more complicated in the later members of the race, and the peculiar markings developed by the repeated partitions provide a convenient method of identification. The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes now living, comprise another group of carnivorous mollusks. These, however, had lost the external shell, and the usual fossil is part of an internal shell or “skeleton,” known as the guard. This limy structure has the form of a pointed cigar, and is seldom over a foot long although the total body-length of the larger animals was commonly about six to eight feet. Several hundred species have been described, the majority being of Jurassic age. They declined rapidly toward the close of the era. The invertebrate life of the Mesozoic was strongly dominated by mollusks, with cephalopods in first place, the bivalve pelecypods and the single-valve gastropods or snail-like forms sharing subordinate positions. The dominating trilobites, sea-scorpions, and tetracorals of the Paleozoic had disappeared, while the brachiopods and crinoids were greatly modified and more like the forms which live today. Crinoids became moderately abundant at various times, but in many ways different from their relatives of the preceding era. Some of the largest known species, with stems estimated as fifty feet long, have been found in lower Jurassic rocks. A great abundance of microscopic life is indicated by the frequent occurrence of chalk in the Cretaceous formations. Corals of the modern reef-building type (hexacorals) were common in the warm seas of a large part of the world.