CONCLUDINa StJMMAEY EXTENT OF EANGE OF VAEIATION. 
573 
whorls as to form a vertical linear axis ; and we find this axis in OrUculina sometimes 
equalling in length the diameter of the spire, so that this organism at an early stage of 
its growth may be nearly spheroidal*. Now among the various types of ios^W. Alveolince, 
there are some whose shape, instead of being fusiform like that of the recent type I 
have described, is almost identical with that of a spheroidal Orhiculma ; and the general 
structure of two such organisms will be so nearly identical, that I cannot see any 
ditficulty in referring them to a common original. And when we examine a series of 
such fossil types, we see that they present a wider and wider divarication from the 
OrUculine type in this one particular alone, — that whilst the later growth of Orhiculma 
tends to liken it to the discoidal Orhitolites, that of Alveolina tends to the continual 
elongation of its vertical axis, a difference which the analogies of the Foraminifera 
generally would indicate to be one of far too small account to be fairly adopted as a 
ground of original distinction f. 
243. In the assemblage of forms which I have thought myself justified in re-assem- 
bling under the designation Peneroplis, we encounter another remarkable series of 
variations, the principal of which have given occasion to the formation of the two 
additional genera Pmdritina and S^pirolina. With an exceedingly close conformity in 
the texture and hi the superficial markings of their shells, as well as in their general 
plan of growth, we observe a marked diversity in the form and proportions of the spire, 
especially in the later stages of its growth, and a still greater.divergence in the form and 
disposition of the septal apertures. For in the type to which M, d’Oebigny restricts 
the generic designation PeneropUs^ we usually find the spire rapidly widening and 
becoming proportionally compressed in each succeeding convolution ; whilst in that 
which he distinguished as Dendritina, the spire widens but slowly, whilst increasing 
rapidly in turgidity. Further, in the one type as in the other, the later extension is 
often in a straight line, instead of continuing to follow the spiral course ; and on this 
variation alone, which is of no account whatever among Foraminifera (as will presently 
appear, nr 255), has been erected the genus SpiroUna. Now in the typical Pmeroplis, 
the septal plane presents a linear series of minute rounded pores ; whilst in the typical 
Pendritina Ave find in their place a single large orifice Avith radiating extensions ; the 
difference betAveen these tAVo modes of communication being as great as we find betAveen 
almost any tAvo types of Foraminifera whatever. Yet I believe that no one aaIio Avill go 
through the details of the evidence I have collected from the study of transitional forms, 
will have any doubt that Peneroplis and Pendritina have had a common progenitor, 
and that the peculiarity in the mode of septal communication that characterizes each is 
intimately related to the compressed or turgid form of the spire in either case ; AAdiilst 
the different forms of the Spirolina type, among Avhich Ave find the most remarkable 
* See Philosophical Transactions, 1856, Plate XXVIII. fig. 8. 
t In this view of the relation of Alveolina to Orhiculina I am supported by Messrs. Pabkee and Eupeet 
J OXE3, who remark that Alveolina “ may be said to represent a small thick OrUculina drawn out trans- 
versely at its umbilici.” — Ann. of Nat. Hist., March 1860, p. 182. 
