PENEROPLIS; ORBICULINA 803 



Reverting again to the primitive type presented in the simple 

 spiral of Cornuspira, we find the most complete development of 

 it in Peneroplis (fig. 606), a very beautiful form, which, although 

 not to be found on our own coasts, is one of the commonest of all 

 Foraminifera in the shore-sands and shallow-water dredgings of 

 warmer regions. This is normally a nautiloid shell, of which the 

 spire flattens itself out as it advances in growth. It is marked 

 externally by a series of transverse bands, which indicate the posi- 

 tion of the internal septa that divide the cavity into chambers ; and 

 these chambers communicate with each other by numerous minute 

 pores traversing each of the septa, and giving passage to threads 

 of sarcode that connect the segments of the body. At a is shown 

 the * septal plane ' closing in the last-formed chamber, with its single 

 row of pores through which the pseudopodial filaments extend them- 

 selves into the surrounding medium. The surface of the shell, 

 which has a peculiarly ' porcellanous ' aspect, is marked by closely 

 set atrice that cross the spaces between the successive septal bands ; 

 these markings, however, do not indicate internal divisions, and are 

 due to a surface-furrowing of the shelly walls of the chambers. This 

 type passes into two very curious modifications, one having a spire 

 which, instead of flattening itself out, remains turgid, like that of a 

 Nautilus, having only a single aperture, which sends out fissured 

 extensions that subdivide like the branches of a tree, suggesting the 

 name of Dendritina, the other having its spire continued in a rec- 

 tilineal direction, so that the shell takes the form of a crosier, this 

 being distinguished by the name of Spirolina. A careful examinar 

 tion of intermediate forms, however, has made it evident that these 

 modifications, though ranked as of generic value by M. d'Orbigny, 

 are merely varietal, a continuous gradation being found to exist 

 from the elongated septal plane of the typical Peneroplis, with its 

 single row of isolated pores, to the arrow-shaped septal plane of 

 Dendritina, with all its pores fused together (so to speak) into one 

 dendritic aperture, and a like gradation being presented between 

 the ordinary forms and the ' spiroline ' varieties, with oval or even 

 circular septal plane, into which both Peneroplis and Dendritina 

 tend to elongate themselves. 



From the ordinary nautiloid multilocular spiral we now pass to 

 a more complex and highly developed form, which is restricted to 

 tropical and subtropical regions, but is there very abundant that, 

 namely, which has received the designation Orbiculina (fig. 606). 

 The relation of this to the preceding type will be best understood 

 by an examination of its earlier stage of growth ; for here we 

 see that the shell resembles that of Peneroplis in its general form, 

 but that its principal chambers are divided by ' secondary septa ' 

 passing at right angles to the primary into ' chamberlets ' occupied 

 by sub-segments of the sarcode-body. Each of these secondary 

 septa is perforated by an aperture, so that a continuous gallery is 

 formed, through which (as in fig. 609) there passes a stolon that 

 unites together all the sub-segments of each row. The chamberlets 

 of successive rows alternate with one another in position ; and the 

 pores of the principal septa are so disposed that each chamber-let of 



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