CONCLUDING- SUMMAJBT: — ^EXTENT OE EANGE OE VAEIATION. 
675 
least account. It is in this type that we recognise the nearest approximation towards 
such forms as Thalasdcolla, which seem to connect Orhitolites with Sponges 67); 
while the relationship which Orhiculina and Peneroplis have been supposed to bear to 
the ordinary HMicostegues, 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 of affinity. Looking to the evidence already adduced in regard to the 
prevalence of particular modifications of Orhitolites in particular localities 62), and 
to the influence of the geographical distribution of the Peneroplis type upon the modi- 
fications it presents 138), 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 then’ primary type. 
Orhiculine Type 
f r 
Peneroplis 
Dendritina, Peneroplis 
Dendritina, Spirolina, Peneroplis. 
diverging into 
A, 
Orbiculina 
A. 
r ; 7"^ 
Orbiculina, Alveolina 
Orbiculina, Orhitolites, Alveolina. 
246. Passing on, now, to an essentially different group, that which includes Nummu- 
lites and its allied forms, we find that the relation of the discoidal Oycloclypeus and the 
helicine Heterostegina is of essentially the same nature with that of Orhitolites and 
Orhiculina (ifir 113, 116); 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. From the 
rarity of Oycloclypeus^ all the known specimens of which 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 afiirmative evidence is obviously not equivalent to a disproof of what has strong analogy 
in its favour. — The variations which I have described among the different forms of Oper- 
culina, 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 difierential characters develope themselves in 
this type ; and a strong probability is thus afibrded in favour of the varietal character 
of larger differences among individuals whose conditions of existence are very diverse. 
