usually lost after the chambered shell grows out from it, but in the aramonoids the protoconch is hard and calcareous and is always found at the narrow end of the series of chambers composing the shell.
The undisputed facts are that in Silurian times straight conical shells with smooth outer surface were common, and coiled shells were rare. In the Carboniferous the coiled shells gain in number and nodules, ribs or keels begin to develop upon their outer surfaces. In the Jurassic we find only close-coiled, or uncoiling shells, and those that uncoil tend to become straight with smooth surfaces, as were their ancestors long ago in Silurian times. The ammonoids arose as coiled forms from the nautiloids in Cambrian times, but in the Jurassic and Cretaceous they uncoil and thus resemble their straight-shelled nautiloid ancestors, the oldest of their race. Uhlig, Neumayr, Zittel, Hyatt and all other students of the group agree upon these points, and indeed Hyatt's observations of fact have won high respect for both their accuracy and their number. It is only in matters of inference that he is at variance with many zoologists.
Hyatt found that in the very young ammonoids the shell is at first straight and smooth, then as growth proceeds it coils upon itself, and may acquire a keeled, ribbed or nodular surface. Finally, if the species be a Jurassic or Cretaceous form it is apt to uncoil in later life, and the uncoiled part of the shell tends to become smooth and relatively straight like its own young stage. D'Orbigny observed these facts even before Hyatt, and they are well authenticated by numerous students of the group. Hyatt, however, pointed out the interesting fact that there is a parallelism between the growth-stages of the individual, and the genetic succession of species through which the race has developed. For example, the young shell is smooth and straight as were the adult shells of its primitive nautiloid ancestors of Cambrian times. The adult shell is coiled and ornamented as were those of the ammonites of the Devonian when the race was dominant. The old shell is again straight and smooth as were its Cambrian ancestors and the vanishing remnants of the race that died out in upper Cretaceous times. Thus the growth-phases of the individual—embryonic, larval, adolescent, adult and old age—are correlated with the changes which occur in the geological history of the group to which it belongs.
It may be a mere coincidence, but certainly in ammonites there is a surprisingly close correspondence between the growth-stages of the individual and the phylogeny of its race. Hyatt believes that this fact is not due to accident, but that the life of the individual and the life of the race are related and obey one and the same law.
Zoologists can not understand why this must necessarily be so, but there are many laws in nature which man has discovered, and the logical necessity for which we have not yet understood, and its incomprehensibility has naught to do with the truth or falsity of Hyatt's theory.