508 



FORAMINIFERA I POLYSTOMELLA. 



the external openings of those on its anterior margin being in the 

 furrows between the retral processes of the next segment. These 

 Canals appear to be occupied in the living state by prolongations 

 of the Sarcode-body ; and the Diverging branches of those of each 



Fig. 256. 



Internal cast of Polystomella craticulata : — a, retral processes, 

 proceeding from the posterior margin of one of the segments ; 

 b, b l , smooth anterior margin of the same segment ; c, c 1 , stolons 

 connecting successive segments, and uniting themselves with the 

 diverging branches of the meridional canals ; d, d 1 , d 2 , three 

 turns of one of the Spiral canals ; e,e l , e 2 , three of the Meridional 

 canals ; /, /», / 2 , their Diverging branches. 



convolution unite themselves, when this is enclosed by another 

 convolution, with the stolon -processes connecting the successive 

 segments of the latter, as seen at c x . There can be little doubt 

 that this remarkable development of the Canal-system has refer- 

 ence 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 which fills-up with a solid ' boss ' what 

 would otherwise be the depression 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 

 PL xv., fig. 16 ; the Umbilical boss in this species, however, being 

 much smaller in proportion than it is in P. craticulata. — There is 

 a group of Foraminifera to which the term Nonionina is properly 

 applicable, that is probably to be considered 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 



