. Fig. 19. Ammonites Iluniphresianus. A form analogous to the Planulata. the shell ; that is to say, it gradually takes possession of the central turns also, as we trace the forms to higher strata. This reproduction in younger stages of life of modifications first occurring at a more advanced age, makes but slow progress, so that we see the older forms repeated with great persistency in the central turns. Frequently a modification of this sort has taken posses- sion of only a small part of the convolutions, when a new one already appears at the outside, and follows the

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