cognize the nearest approximation towards such forms as Thalassicolla, which seem to connect Orbitolites with Sponges; while the relationship which Orbiculina and Peneroplis have been supposed to bear to the ordinary Helicostègues, being dependent only on plan of growth, and being utterly at variance with the essential characters of the two groups, must be regarded as one of analogy, not affinity. Looking to the evidence I have adduced in regard to the prevalence of particular modifications of Orbitolites in particular localities, and to the influence of the geographical distribution of the Peneroplis type upon the modifications it presents, we seem justified in extending the same view to those larger (though not more essential), differentiations which these types must have undergone on the hypothesis of their derivation from the same original. The following may be suggested as the mode in which the existing forms might thus have diverged from each other, and from their primary type.
| ORBICULINA TYPE, | ||
| Diverging into | ||
| Peneroplis. | Orbiculina. | |
| Dendritina, Peneroplis, Dendritina, Spirolina, Peneroplis. |
Alveolina, Orbiculina, Alveolina, Orbiculina, Orbitolites. | |
Passing on, now, to an essentially different group, of which Operculina may be taken as the type, I have shown that the relation of the discoidal Cycloclypeus and the helicene Heterostegina is of essentially the same nature with that of Orbitolites and Orbiculina; the minute structure of the shell and the physiological condition of the sarcode body being essentially the same in the two organisms, and the only important divergence between them being in their plan of growth. Prom the rarity of Cycloclypeus, all the specimens of which yet known have been brought from one locality, I have not yet had the opportunity of ascertaining whether it ever presents in an early stage any approximation to the helical mode of growth; but such a deficiency of affirmative evidence is obviously not equivalent to a disproof of what has strong analogy in its favour.
The variations which I have described (3rd series) among the different forms of Operculina, although limited to the form of the spire, and the character of the surface-markings, would be amply sufficient to justify the erection of numerous species, were it not for the connexion established between the most divergent forms by intermediate links, and the necessity for an almost indefinite multiplication of hypothetical originals which the adoption of such a method would involve. The existence of such a large extent of variation among the specimens collected in the same locality must be admitted as valid evidence of the facility with which differential characters develope themselves in this type; and a strong probability is thus afforded in favour of the varietal character of larger
