490 FOEAMTNIFERA '. — MILIOLIDA ; PENEROPLIS. 



honeycomb division ; and these diversities have been used for the 

 characterization of Species. Not only, however, may every inter- 

 mediate gradation be met-with between the most strongly marked 

 forms, but it is not at all uncommon to find some parts of the 

 surface smooth whilst others are deeply pitted or strongly ribbed or 

 honeycombed ; so that here again the inconstancy of these differences 

 deprives them of all value as distinctive characters. 



373. Reverting again to the primitive type presented in the 

 simple Spiral of Cornuspira, we find the most complete develop- 

 ment of this type in Peneroplis (Plate xv. , fig. 5), a very beautiful 

 form, which, although very rare on our own coasts, is one of the 

 commonest of all Foraminifera in the shore-sands and shallow- 

 water dredgings of the warmer regions of every part of the globe. 

 This is a Nautiloid Shell, of which the spire flattens itself out as it 

 advances in growth ; it is marked externally by a series of trans- 

 verse bands, which indicate the position 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 themselves into the surrounding 

 medium. The surface of the Shell, which has a peculiarly Por- 

 cellanous aspect, is marked by closely-set striae that cross the 

 spaces between the successive septal bands ; these markings, how- 

 ever, do not indicate internal divisions, and are due to a ridge-and- 

 furrow arrangement of the shelly walls of the chambers. This 

 type passes into two very curious modifications ; one having a 

 Spire which remains turgid like that of a Nautilus, instead of 

 flattening itself out, with a single aperture which sends out fissured 

 extensions that subdivide like the branches of a tree, suggesting 

 the name of Dendritina which has been given to this variety; the 

 other having its spire continued in a rectilineal direction so that 

 the shell has the form of a crosier, and being distinguished by the 

 name of Spirolina. A careful examination 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 Peneroplis, with its single row of isolated pores, to 

 the arrow-shaped, oval, or even circular septal plane of Den- 

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

 dendritic aperture ; and a like gradation being presented between 

 the ordinary and the ' spiroline ' forms, into which both Peneroplis 

 and Dendritina tend to elongate themselves under conditions not 

 yet fully understood. 



374. From the ordinary Nautiloid multilocular spiral, we now 

 pass to a more complex and highly- developed form, which is re- 

 stricted to tropical regions, but is there very abundant, — that, 



