302 THE POPULAR SCIENCE MONTHLY 
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 on- 
togeny, 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 Ostrea, 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. Hach 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 ex- 
amples of this are Mulleria, a member of the Unionide—like Anodon 
in the young; Hinnites, 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 remark- 
ably 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 
Terebratellide. In both the boreal and austral subfamilies a very com- 
plete series of genera correspond to the ontogenetic stages of the ter- 
minal or highest genera. Another interesting case is that of Orbicu- 
loidea. This discoid shell has at first a straight hinge like [phidea. 
