THE FORAMINIFERA 107 



regarded as an expansion of the end of the spiral passage. In 

 those forms of duplex in which the spiral passage communicates 

 with more than one chamberlet the end is somewhat expanded. 

 An extension of this expansion round the outer side of the spiral 

 passage would give rise to the complete crescentic chamber which 

 we find in complanata. 1 



Looking back on the series of forms of Peneroplididae hitherto 

 examined, a gradual increase in complexity of structure is to be 

 observed. We pass from Peneroplis, with undivided chambers dis- 

 posed at first on a spiral and often, later, on a rectilinear plan, to 

 Orbiculina, with subdivided chambers similarly disposed, though in 

 one variety of the megalospheric form the annular arrangement 

 is assumed. In Orbitolites marginalis the chambers and pores are 

 disposed in a single plane, and in the early stages of growth we 

 find arrangements repeating in some of their features those of 

 Peneroplis and Orbiculina before the annular arrangement which is 

 characteristic of Orbitolites is arrived at. 0. duplex, with its double 

 series of pores, furnishes an intermediate stage to the complex 

 three-layered condition of 0. complanata. 



In his Report on the genus Orbitolites, Carpenter made this 

 series of genera and species the subject of a " Study of the Theory 

 of Descent," and laid stress on the remarkable manner in which 

 the forms of the simpler members are repeated in the life-history 

 of the more complex. When this Report was published (1883) 

 attention had only recently been drawn to the phenomenon of 

 dimorphism in the Foraminifera, and Carpenter does not appear 

 to have been aware of the existence of the microspheric forms, as 

 constituting a distinct set of individuals. 



On comparing the mode of growth of the microspheric and 

 megalospheric forms, we find a contrast between them comparable 

 to that presented by the Miliolinidae. While the microspheric 

 forms repeat successively the shapes and arrangements of chambers 

 which are permanent in other, and in this case, simpler, members 

 of the group, in the megalospheric forms these stages are to a 

 greater or less extent abridged or altogether omitted. Thus in 

 the megalospheric form of Orbitolites marginalis the peneropline 

 series of single chambers which succeeds the spiral canal is fewer 

 in number than in the microspheric form, but the orbiculine 

 arrangement is well represented. In this form of 0. duplex the 

 peneropline condition has almost or entirely disappeared, and the 

 orbiculine stage is much abbreviated. In 0. complanata both 



1 The remarkable fossil form Meandropsina described by Schlumberger (59) 

 appears to be related to Orbitolites, the surface of the disc being covered with a 

 layer of chamberlets arranged in a Meandrina-like manner. Schlumberger finds both 

 microspheric and megalospheric forms are represented in his specimens. 



