54 OF THE FORAMINIFERA GENERALLY: 



affinities to a belief in their mutual affinity. A most complete isomorphism presents itself, 

 for example, in the simple spiral which constitutes one of the lowest forms of the porcellanous, 

 the arenaceous, and the hyaline series respectively ; and by such as have not traced out the 

 affinities of this spiral in each series, it might readily be supposed that such a conformity in 

 the plan of growth of the shell must be indicative of conformity in the physiological condition 

 of the animal.* But the porcellanous Cornuspira and the hyaline Spirillina differ not merely 

 in the texture of their shells, but in what we have seen to be the fundamental character of 

 non-perforation in the one and perforation in the other ; and whilst the former graduates 

 almost continuously into the spiroloculine form of Miliola, the latter is in like manner related 

 to Botalia through the vermiculate varieties of the latter genus. So the arenaceous, simply 

 spiral Trocliammina, which agrees with Conimpira in its non-perforation, not only differs from 

 it in the almost purely arenaceous composition of the shell, but also in the varietal forms into 

 which it passes ; and by some of these it is brought into relation with the other types of the are- 

 naceous series. — Passing now to the higher forms, we meet with so remarkable an isomorphism 

 between the nautiloid forms of Peneroplis and Operculina (the septa of the latter genus, besides 

 their regular aperture at the inner margin, being perforated here and there by secondary pores 

 that remind us of the former), between the subdivided spirals of Orbiculiiia and Heterostc^ina, 

 and between the discoidal OrbitoUtes and Cydoclyiwus, that, in any classification founded mainly 

 upon plan of growth, the genera of each pair must be placed in near proximity to each other ; 

 yet they are really separated by all those characters which have been shown to possess the 

 highest physiological value, and can only be regarded as " representing " each other in the 

 series to which they respectively belong. A like " representation '' will be shown to exist 

 among the higher forms of the arenaceous series ; certain varieties of Lituola, for example, 

 bearing a close resemblance to Nonionina, and others to the spiroline variety of Peneroplis ,• 

 whilst ValvuJina may take on the forms of Ttotalia, BuUuiina, and other hyaline Foraminifera. 



67. It seems obvious, from the foregoing considerations, that the importance of plan of 

 growth, as a character available in the classification of Foraminifera, is far below tliat of the 

 aggregate of other characters which stand in more intimate relation to the physiological con- 

 dition of the animal ; and the low value which ought to be attached to it is further indicated 

 by its frequent tendency to variation within the limits of what are shown by the evidence of 

 gradational affinity to be well-marked natural groups. Such a tendency seems greatest in 

 the lower types of each series ; " polymorphism " being the rule among them, rather than the 

 exception. Thus we shall find that Niibecularia, one of the simplest of the porcellanous 

 Foraminifera, presents itself under such a variety of forms, that the attempt to classify these 

 in any system based on the geometrical arrangement of the successive segments, would 

 lead to nothing but a most absurd separation of what are clearly but varietal modifications of 

 one and the same type. In Vertebralina we find, with a closer general conformity to a 

 common type, a range of variation which is still very remarkable ; and even when we rise as 



* Thus, by Professor Scliultze (xcvii, pp. 40, 41), the porcellanous and the hyaline spirals have been 

 ranked as Cornuspira jjlanorbis and C. perforata ; whilst by Prof. Williamson (cs, pp. 92, 93), the 

 porcellanous, arenaceous, and hyaline spirals are designated respectively Spirillina foliacea, S. arenacea, 

 and S, perforata. 



