42 



THE CYSTIDEA 



suppose that the canals represent stroma strands continuous 

 across the sutures ; those crossing any one suture come to occupy 

 a rhombic area bisected by the suture-line, and, since, in weathered 

 plates, there appear to be pores at the ends of these canals, the 

 areas have been called " pore-rhombs " (Poren-raiiten, see Fig. XV., 

 Echinosphaera, and XVI., Orocystis). There also takes place a gradual 



Fio. I. 



Structure of the test in Aristocystidae. 1, plate of A ristocystis boltemicns showing haplopores, 

 some of which are connected by a horse-shoe canal, x 4 ; 2, portion of same further enlarged ; a. 

 portions of surface of other specimens, gradually leading up to such a structure as in 4, a plate 

 of Calix Sedgwicki, with diplopores ; 5, a diplopore of rather different form ; 6, section of a 

 diplopore ; the hypostereom is shown, but the epistereom, if there were any, is removed ; 7, a 

 natural replacement (by infiltration of mineral matter) of the original stroma-strands and 

 sutures in plates of Calix (?), the stereom dissolved away ; 8, plates of Aristocystis in similar 

 condition showing vertical strands ( = haplopores) in the middle, and radiating strands at the 

 sutures. All figures enlarged. (1, 2, 3, and 8 after Barrande ; 4 and 7 after Renault.) 



increase in the area, and a decrease in the number, of the thecal 

 plates relative to the size of the theca ; perhaps the folds that 

 often radiate from the urn bo of each plate are connected with this, 

 for they must have strengthened the plates, like the folds in cor- 

 rugated iron or pasteboard. These folds may coexist with diplo- 

 pores or with pore-rhombs ; but they are clearly more adapted to 

 the latter structure, and often seem to merge with it and accentuate 



