POLYSTOMELLA 



829 



with the stolon processes connecting the successive segments of the 

 latter, as seen at c 1 . There can be little doubt that this remarkable 

 development of the canal system has reference to the unusual amount 

 of shell -substance which is deposited as an 'intermediate skeleton' 

 upon the layei 1 that forms the proper walls of the chambers, ami 





/, 

 /&&, 



>/$/{%':& 

 . &:'/; .;"-:-. 



FIG. 627. Cycloclyi)eus external surface and vertical and horizontal sections. 



which fills up with a solid ' boss' what would otherwise be the de- 

 pression at the umbilicus of tin- spire. The substance of this 'boss' 

 is traversed by a set of straight canals, which pass directly from the 

 spiral canal beneath, towards the external surface, where they open 

 in little pits, as is shown in Plate XIX, -23, the umbilical boss 

 in P. ci'ispa, however, being much smaller in proportion than it 



FIG. 6'28. Operculina laid open to show its internal structure : , marginal 



general distribution of which is seen in the septa e, e', the lines radiating 

 from c, c point to the secondary pores ; g, g, non-tubular columns. 



is in P. craticulata. There is a group of Foraminifera to which the 

 term Xoiiionina is properly applicable, that is probably to be con- 

 sidered as a sub-genus of Pob/stoineU<i. agreeing with it in its general 

 conformation, and especially in the distribution of its canal system, 

 but differing in its aperture, which is here a single fissure at the 

 inner edge of the septa 1 plane, and in the absence of the ' retral 



