PRINCIPLES OF CLASSIFICATION. 55 



high as Peneroplis, we very commonly observe a change in the direction of growth from the 

 spiral to the rectilineal with the advance of life. The spiral growth of OrbicuUna, again, 

 often gives place to a cyclical plan exactly resembling that which is typical of Orbitolitcs ; 

 whilst at the commencement of the development of Orbitolites, which is typically cyclical, the 

 chambers are sometimes added according to a spiral arrangement. In the same manner, when 

 we enter upon the " hyaline" series, we shall find ourselves compelled by the like continuity 

 of affinities to rank as varietal modifications of the single generic type Nodosaria a long list 

 of reputed genera, separated by D'Orbigny under his three orders, Sfichostet/ues, Helicoste^ues, 

 and Enallosteyues : and marked changes in the plan of growth frequently present themselves 

 among the higher genera of that series in the life of one and the same individual ; the early 

 arrangement of the chambers of PlanorbuJina and Tinoporm, for example, being as typically 

 spiral as that which prevails in Bofalia, but flie additions being subsequently made in such a 

 manner as to convert the spiral, in the first instance, into a circular disk, which may then 

 increase at its periphery so irregularl}' that all definiteness of contour vanishes, whilst the 

 chambers may also be piled one upon another in an irregular " acei'vuline" manner, so as 

 entirely to mask either their original spiral or their secondary cyclical arrangement. 



68. The foregoing examples serve to show, not only that neither plan of t/rowth nor 

 remdtant form can be rightly taken as a character for sepai-ating the great primary divisions 

 of FoRAMiNiFERA, but also that they are so often liable to variation within the limits 

 of genera, that no constant reliance can be placed on them as means of difl'erentiating even 

 these subordinate groups from each other. It will be usually found much safer, in fact, to 

 place our chief reliance on those characters which can be stated in terms of each individual 

 segment, than on those which can only be predicated of the aggregate. And no characters 

 are, in general, so free from the fallacy resulting from tendency to variation, as those which 

 are drawn from the nature and position of the septal apertures. Even these, however, in 

 certain exceptional cases, share in the general tendency to variation ; and in estimating the 

 value which should be attached to such diversities, it is important to bear in mind the remark 

 already made (p. 8) as to the purpose which is served by these apertures. It must obviously 

 be a matter of no great physiological importance, whether a number of those fine threads of 

 sarcode, which act as stolons connecting the successive segments of the body, and are put 

 forth as pseudopodia from the last, pass out in one undivided bundle, or be separated by the 

 interposition of minute pi'ocesses of shell, which convert a narrow fissure into a row of pores, 

 or a wide orifice into a cribriform plate. Such a difference exists between the aperture of 

 Rotalia and that of Calcarina, and between that of Miliola and that of Haucrina ; and it could 

 not be regarded as even of sub-generic value in those two cases, if it were not accompanied 

 by other distinctions. A far greater dissimilarity exists between the aperture of Feneroplis 

 and that of Bendritina, the former consisting of a linear series of separate pores, whilst the 

 latter is a single, large, dendritic orifice : and yet, as I have elsewhere shown (xv), the former 

 of these conditions graduates into the latter so continuously as to render it impossible to draw 

 any definite line of demarcation between them ; and each is related to the shape of the septal 

 plane, which may vary no less gradationally from that of a long, narrow band (Plate VII, 

 fig. 16, ff), to a cordate or sagittate form (figs. 6, a, 14, h, c), according to the compression or 

 turgidity of the spire. Hence, as there is a most remarkable accordance between these two 



