CARPENTER ON FORAMINIFERA. 191 



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 or- 

 dinary Helicostegues, 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 geographi- 

 cal 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 under- 

 gone 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. 



_A__ 



Dendritina, Peneroplis, Alveolina, Orbiculina, 



Dendritina, Spirolina, Alveolina, Orbiculina, 



Peneroplis. Orbitolites. 



Passing on, now, to an essentially different group, of which Oper- 

 culina 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 essen- 

 tially the same in the two organisms, and the only important divergence 

 between them being in their plan of growth. Prom the rarity of Cyclo- 

 clypeus, 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 cha- 

 racter of the surface-markings, would be amply sufficient to justify the 

 erection of numerous species, were it not-ibr the connexion established be- 

 tween 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 lo- 

 cality must be admitted as valid evidence of the facility with w r hich 

 differential characters develope themselves in this type ; and a strong 

 probability is thus afforded in favour of the varietal character of larger 



