232 THE FOEAMINIFEEA 



To turn to the separate divisions of this family, 

 the sub-family Fusulinin.e has the important genus 

 FusuUna for its type. This, with the various allied 

 genera and sub-genera, are fossil forms, and restricted 

 to palaeozoic strata. Their shell-structure is not 

 typically nummuline, for the canal system and sup- 

 plementary skeleton appear to be absent. 



The next group of the Polystomellin.e is a farther 

 step towards the higher shell- structure seen in the 

 Nummulites. In Nonionina the shell-wall is still 

 simple in character, but the suture lines are some- 

 times thickened by an extra shell-layer. Pohjstomella 

 comprises an instructive series of shells in which the 

 graduations may be traced from those w^ith a simple 

 series of interseptal canals opening out at the suture 

 lines by a row of pores, to those with a more compli- 

 cated system, and with a varying amount of exogenous 

 shell-structure forming a true supplemental skeleton, 

 as well as secondary thickening of the test in the 

 form of septal bridges laid at intervals across the 

 sutural furrows, and also sutural costte. 



The genus ArclicBcliscus represents the simplest 

 form of the typical nummulite group, in being little 

 more than a simple non-septate or partially septate 

 tube wound round an axis which at intervals changes 

 its direction, as in many of the milioline genera. This 

 genus, which is restricted to Carboniferous rocks, 

 shows an affinity with the nummuline forms in 

 having a thick laminated shell. It is finely porous, 

 but also exhibits a secondary kind of tubulation in 

 the form of numerous coarser pores or canals. Am- 



