PROTOZOA: FORAMINIFERA. 7 1 



the anterior wall of the preceding segment, so that each septum ("septal 

 plane " ) is composed of two lamellae, as seen in fig. 13, A (Carpenter). 

 Moreover, "in the higher types of the hyaline or vitreous series we 

 frequently meet with an ' intermediate ' or ' supplemental ' skeleton, formed 



Fig. 13. A, Diagram of one of the higher forms of the vitreous Foraminifera, showing 

 the double nature of the septa (b), the stolon-passages between successive chambers 

 (a), and the supplemental skeleton (d) ; B, Test of Calcarina Spengleri, magnified 

 twelve diameters, showing the spines formed by the supplemental skeleton ; C, Part 

 of a section of the test of Cakariua, magnified fifty diameters, showing the tubulated 

 " proper walls" of the chambers (), and the canal-system of the intermediate skeleton 

 (cf) ; D, Part of the test of Nummulina l&vigata, highly magnified, showing the 

 canal-system of the septa (s), and marginal cord () (After Carpenter.) 



by a secondary or exogenous deposit upon the outer walls of the chambers, 

 by which they receive a great accession of strength. This deposit not only 

 fills up what would otherwise be superficial hollows at the junctions of the 

 chambers (fig. 13, A, d), or (as in Polystomella) at the umbilical depression, 

 but often forms a layer of considerable thickness over the whole surface, 

 thus separating each whorl from that which encloses it ; and it is some- 

 times prolonged into outgrowths that give a very peculiar variety to the 

 ordinary contour, as in some varieties of Rotalia and Polystomella, but 

 most characteristically in Calcarina (fig. 13, B). This intermediate or sup- 

 plemental skeleton, wherever developed to any considerable extent, is 

 traversed by a set of ' canals, ' which are usually arranged upon a syste- 

 matic plan, and are sometimes distributed with considerable minuteness " 

 (Carpenter). The canals of this system are doubtless filled in the living 

 state by prolongations of the sarcode, which serve to keep up the vitality 

 of the intermediate skeleton. This intermediate skeleton, with its canal- 



