Page:Popular Science Monthly Volume 77.djvu/308

302 the nepionic stage, before the whorls come into contact. It has been carried back in the ontogeny by acceleration. Smith concludes from a study of the development and phylogeny of Placenticeras, an Upper Cretaceous ammonoid, that "the development of Placenticeras shows that it is possible, in spite of dogmatic assertions to the contrary, to decipher the race history of an animal in its individual ontogeny."

Among the Gastropoda, Grabau and Burnett Smith have pointed out numerous beautiful cases of recapitulation. In Fusus and its allies, the higher forms quite constantly resemble in their earlier stages the adults of ancestral forms. Even in profoundly modified gerontic types, the young resemble the ancestors. Smith has brought to light in Athleta (Volutilithes) of the Eocene, an almost perfect example of even and regular acceleration, with its correlative, the recapitulation in the young of the Upper Eocene forms of the adult characters of the Lower Eocene forms. The stages passed through by this group of shells are, beginning with the earliest, a smooth, curved rib, cancellated, spiny and sometimes a senile stage. In the ancestral species (A. limopsis) the curved rib stage comes in at the close of the fourth whorl, whereas in the Upper Eocene form (A. petrosa), this stage comes in at the beginning of the third whorl.

Among the Pelecypoda the classic researches of Jackson are familiar to all. He shows that the modern Pecten passes through, in its ontogeny, a series of stages resembling adult Rhombopteria, Pterinopecten and Aviculopecten, and that the geologic order of these genera is the same as the ontogenetic order in Pecten. In such monomyarian genera as Ostrca, the initial shell, or prodissoconch, is dimyarian, and resembles the primitive Nucula. Again, in various more or less widely separated genera, the condition of complete cemented fixation has produced the ostreaform shape. Each one of these genera, however, except where the modification of shape due to fixation appears very early in ontogeny, recapitulates the adult characters of its respective ancestor. The examples of this are Mulleria, a member of the Unionidæ—like Anodon in the yo ung; Einnites, a member of the Pectinacea—like Pecten in the young; Spondylus, another member of the Pectinacea—like Pecten in the young.

Beecher's various studies of the Brachiopoda not only brought out the fact that the initial shell or protegulum of the brachiopod is remarkably similar to the most primitive known Lower Cambrian brachiopods, but have supplied in addition numerous other remarkable examples of recapitulation. One of the most striking of these is the case of the Terebratellidæ. In both the boreal and austral subfamilies a very complete series of genera correspond to the ontogenetic stages of the terminal or highest genera. Another interesting case is that of Orbiculoidea. This discoid shell has at first a straight hinge like Iphidea.