Page:Popular Science Monthly Volume 77.djvu/309

Rh It next resembles Obolella, then at a later stage it is like Schizomania, and finally adult growth brings in the characters of Orbiculoidea. Raymond has shown the remarkable similarity of the neanic stage of Spirifer mueronatus to the adult S. crispus of the Niagara. Shinier and Grabau found in the upper Hamilton of Thedford, Ontario, a variety of Spirifer mueronatus that is very mucronate in the young and not at all so in the adult. The derivation of this form from S. mueronatus is beyond question. I have pointed out a precisely similar case in Platystrophia acutilirata var. senex. This variety, which occurs in the upper Whitewater beds of Indiana and Ohio, has a hinge angle of nearly 90° in the adult. In the young, however, the outlines of the shell are exactly like the typical P. acutilirata, from which it is beyond any question descended. Greene has shown that Chonetes granulifer of the Carboniferous is, in the neanic stage, like the Devonian Chonetes, and that the hinge-spines come in at a considerably earlier stage in the Carboniferous than in the Devonian and Silurian forms, showing the acceleration of this character.

In the Bryozoa I have pointed out the fact that the colony behaves as an individual, and like an individual recapitulates in its ontogeny (astogeny) ancestral characters. This is beautifully shown in Fenestella, in which the earlier zooecia are strikingly like the adult zoœcia of the Cyclostomata. The adolescent zocecia of Devonian Fenestella are similar to the adult zocœia of Niagara forms. Lang has brought together numerous cases of recapitulation among Jurassic and Cretaceous Stomatopora and Proboscina. The method of dichotomy in the earlier portions of the colony is constantly more like the normal dichotomy of ancestral species.

In graptolites the remarkable researches of Ruedemann clearly indicate that the graptolite colony recapitulates ancestral characters, the proximal thecæ being similar to ancestral adult thecæ. He says:

The rhabdosomes in toto and their parts, the branches, seem also to pass through stages which suggest phylogenetically preceding forms.

Among the trilobites the studies of Beecher, Walcott and Matthew are classic. Beecher has shown that there is a common larval form, the protaspis, and that in higher genera characters appear in the protaspis that are known only in the adults of more primitive genera. For example, the "main features of the cephalon in the simple protaspis forms of Solenopleura, Liostracus and Ptychoparia are retained to maturity in such genera as Carausia and Acontheus." Larval Sao has characters that occur in the adult of Ctenocephalus. The larval stages of Dalmanites and Proetus have characters that appear only in the adult of ancient genera.

Among the corals Beecher and Girty show that such genera as Favosites have early stages that suggest Aulopora. Lang, in a recent