AMMONITES, am'o- atez, the general name for the fossil cephalopod mollusks of the order Ammonoidea, given originally because of a fancied resemblance of the coiled specimens first known to a ram's horn, the symbol of Jupiter Ammon. Subsequently it served as a generic name for a group, but this has been abandoned in the light of later information. The Ammonoidea are one of the two orders of chambered tetrabranchiate cephalopods, the other being the Nautiloidea (see NAUTILUS). Their remains are found fossil in marine Palxzoic rocks from the Devonian to the close of the Mesozoic Age. More than 5,000 species have been described, grouped into about 100 families, and these into nine sub-orders in two divisions, (1) Intrasiphonata and (2) Extra siphonata. The first group contains a single primitive (Devonian) sub-order having the siphuncle dorsally situated; the second contains all the remainder, which agree in having the siphuncle ventral. The classification is based upon the complexly lobed pattern of the sutures, or lines of union between the septa or partition walls of the chambers and the outer wall of the shell (See CEPHALOPODA). The shells of am monites were typically coiled in a single plane and ran in size from an inch or two in diameter to two or more feet; but this varied greatly, even to partial or entire straightness. The sur face of the shell, too, was in many cases smooth and polished or slightly ridged, while in others it was roughly ringed or covered with cross lines, spikes and tubercles, in handsome variety. Some shells were so compressed as to have the proportions of a watch; while others were almost globose.
As Hyatt states, ammonoids experienced a progressive evolution from the early Devonian until the upper Juras, when the group reached its summit of importance and was represented in great numbers and variety in all parts of the world: that is, when it attained the summit of its evolution in complexity of structure, form and ornament. Ammonoids exist in great abundance in the rocks of this period in the western United States, especially those of the irregular group called Ceratites, which suc ceeded the Palaeozoic Goniatites, and other primitive forms. The Jurassic ammonoids show a mixture of retrogression with some progressive features. "Part of their losses are regained by the evolution of the vast number of forms and modifications during this period, but there are numerous localized signs of retro gression, due perhaps to unfavorable surround ings.° Indications of this kind occur sporad ically throughout the Jurassic time and become general in the Cretaceous period. Many of the later forms were openly or grotesquely coiled, or coiled only when young, becoming nearly or quite straight when they grow older, as in Ptychoceras, Turriteles, Scaphites, etc. These degraded aold-age° types were evidently due to the waning forces of life or to disease, because similar though much less marked uncoiling of shells, due to unfavorable condition of the water, have been observed in the fresh-water Planorbis of Steinheim, Germany, and else where. (See EVOLUTION; SENESCENCE). Hyatt
thus infers that there was in the European seas, at least, a widespread unfavorable change in their physical surroundings, asimilar to but more extensive than that which affected Euro pean forms during the Lower Oolite,° and to this influence he ascribes the uncoiling of the shells of Spiroccras and its allies. At the close of the Cretaceous period the ammonoids en tirely disappeared. We thus see in the vast and more or less complete and continuous series of these beautiful shells, in which the imper fections of the geological record are less marked than in other groups, the process of rise, cul mination, decline, and death of a type, present ing also beautiful illustrations of the biogenetic law (q.v.). The type begins with infantile and larval forms, then evolves youthful, mature, and finally old age forms, which present in their simple and closely coiled shells a return to the original simplicity of the infancy and childhood of the type.
Concerning the animals which made the shells, nothing is known except by inference. The growth of the shell begins with the forma tion of the primitive conically-shaped shell called aprotoconch,° and then the secondary shell begins to grow and becomes coiled up in one plane. Like the nautilus the mollusk lived in.the outer chamber of its shell, from which it periodically advanced. The aperture of outer chamber was closed when the withdrew into it, either by a single horny pia (anaptychus) or by a pair of calcareous plate (aptychus). The very earliest appear to ha; been swimmers, like the nautiloicts; but th great bulk of the ammonites undoubtedly gregariously alongshore, where they crawie about, carrying or partly dragging their and searching for the animal food upon whirl they subsisted. The learned Zittell points that their shells were proportionately less be& than those of the nautiloids, and ingly less buoyant; and the probability is tha: they swam little and were rarely active. Ths- is reason to believe that in the case of se species the eggs were retained within the shel ter of the living-chamber until they hatchet:, and that the young remained there until soot what grown. See GONIATITES.
Cook, (Cambridge Narari History. Mollusks' (1895); Zittell-Eastraz. of Paleontology,' Vol. Hyatt, of the Anetidm) (Menacin Mus. Comp. Zool., Vol. XVII, 1889) ; eny of an Acquired Characteristic' (P7T•. Amer. Philos. Society, Vol. XXXII, No. 143. 1894) ; Wurtenberger, iiber die Stan mesgeschichte der Ammoniten' (Leipzig and numerous other papers; also articles h D'Orbigny, Neumayr, Pumpelly, QuensteL Sandbcrger, .Suess, Waagen, White, Whiteays Wright and Zittel.