CONCLUDING STJMMAKY: EXTENT OF EANGE OF VAEIATION. 573 



whorls as to form a vertical linear axis ; and we find this axis in Orbiculina 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 fossil 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 Orbiculina ; and the general 

 structure of two such organisms will be so nearly identical, that I cannot see any 

 difficulty 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 

 Orbiculine type in this one particular alone, that whilst the later growth of Orbiculina 

 tends .to liken it to the discoidal Orbitolites, 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 Dendritina and Spirolina. With an exceedingly close conformity in 

 the texture and in 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 Peneroplis, 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, ^[ 255), has been erected the genus Spirolina. Now in the typical Peneroplis, 

 the septal plane presents a linear series of minute rounded pores ; whilst in the typical 

 Dendritina we find in their place a single large orifice with radiating extensions ; the 

 difference between these two modes of communication being as great as we find between 

 almost any two types of Foraminifera whatever. Yet I believe that no one who will go 

 through the details of the evidence I have collected from the study of transitional forms, 

 will have any doubt that Peneroplis and Dendritina 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 ; whilst 

 the different forms of the Spirolina type, among which we find the most remarkable 



* See Philosophical Transactions, 1856, Plate XXVIII. fig. 8. 



t In this view of the relation of Alveolina to Orliculina I am supported by Messrs. PAKKEU and EUPEET 

 Jo^E8, who remark that Alveolina " may be said to represent a small thick Orliculina drawn out trans- 

 versely at its umbilici." Ann. of Nat. Hist., March 1860, p. 182. 



