POLYSTOMELLA 



829 



with the stolon processes connecting the successive segments of the 

 latter, as seen at c [ . 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 layer that forms the proper walls of the chambers, and 



FIG. 627. C y clod ypeus 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 the 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. crispa, ho\vever, being much smaller in proportion than it 



FIG. 628. Operculina laid open to show its internal structure : a, marginal 

 cord seen in cross-section at a'; 6, b, external walls of the chambers; 

 c, c, cavities of the chambers ; c', c', their alar prolongations ; d, <7, septa 

 divided at d' d' and at d" so as to lay open the interseptal canals, the 

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

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



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

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

 sidered as a sub-genus of Polystomella, 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 septal plane, and in the absence of the ' retral 



