REPORT ON THE GENUS ORBITOLITES. 45 



however, the flattening of the spire is followed by the formation of a complete 

 septum across its mouth, traversed by a series of perforations at regular intervals, as in 

 Feneroplis (fig. 7), another very decided advance in development is marked ; and this 

 might have been followed, as in the preceding case, by a second period of growth upon 

 the newer (PeneropUne) plan. But in our Orhitolites tenuissima we find it constituting 

 only a transition -stage to the next developmental advance, namely, the subdivision of 

 the chambers into Orbiculine chamberlets ; after which, again, development gives place 

 for a time to growth, every addition being a mere multiplication of similar parts. In 

 Orhicidina it seems a matter of indifference whether the later growth co^itinues the spiral 

 of the earlier, or changes to the cyclical plan. But in Orhitolites the spiral is only a 

 transitory phase ; the multiplication of chamberlets producing such a rapid extension of 

 each successive zone, as early to bring about a completion of the annuli, and the establish- 

 ment of that cyclical plan of growth which is the distinctive feature of the Orbitoline ty]3e. 

 When that type has once been reached, the increase of the disk in the horizontal plane to 

 any extent, by the multiplication of its annuli of chamberlets, is a mere process of growth ; 

 but the production of the " complex " type from the " simple " involves, as we have seen, 

 a degree of structural differentiation, which marks a great advance in development. 



And yet, with all this, the physiological condition of the sarcodic body remains (so 

 far as can be made out) essentially the same. The sub-segmented body of the spiral 

 Orhiculina is nourished by the food-particles drawn in through its septal pores, pre- 

 cisely as is the segmented body of Peneroplis ; and the arrangement of its sub-segments 

 in complete annuli, instead of along the expanded mouth of a spire, cannot make any 

 alteration in the mode either of the reception of nutriment, or of its transmission from 

 the peripheral to the central portion of the body. The adult cyclical Orhitolites 

 tenuissima or Orhitolites marginalis must feed in exactly the same manner as it did in 

 its young (Orbiculine) spiral phase ; and the multiplication of the rows of miu-ginal pores 

 in Orhitolites complanata, corresponding with the increased thickness of its disk, merely 

 serves to increase its ingestive capacity, in accordance with the increased require- 

 ments of a more bulky body. The animal of each disk, whatever be its mode of obtain- 

 ing nutriment, can benefit only by the food-particles which come in its way ; and its 

 pseudopodial extensions will draw-in these just as well, whether they issue from one, two, 

 or multiple rows of pores, — ^just as will those of the Peneropline type, whether they issue 

 from the single row of separate passages which traverse the narrow septal plane of the 

 tjrpical Peneroplis, from the partially-coalesced multiple passages of the widened Spiro- 

 lina, or from the single large dendritic orifice formed by the complete coalescence of 

 separate passages in the broad septal plane of Dendritina} 



The external conditions under which the Foraminifera exist are so uniform, except 

 as to temperature and depth of water — which seem to aflect growth rather than develop- 



' See the account of these types given in my Third Memoir, Phil. Trans., 1859, pp. 1-12. 



