PEXEROPLIS; ORBICULINA 803 



Reverting again to the primitive type presented in the simple 

 spiral of Gor/tnHj>/r<>. we find the most complete development of 

 it in Paterojdis (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 

 wanner 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 ti-aversing each of the septa, and giving passage to threads 

 of sarcode that connect the segments of the body. At <i is shown 

 the 'septa! 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 close 1\ 

 set strife 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 

 NoMtilus, 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 examina- 

 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 arrou ^haped 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 /V// '/>/////* and l)<-n<l,-itiiin 

 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 Orfiif/'lnti/ (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 l'<'ii>'r<>j>lis in its general form. 

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

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

 l>y 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 chamberlet of 



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