'108 



FAMILY MILIOLIDA. 



Frequently, however, it happens that the circumambient chamber is partially subdivided on 

 one side by an interposed partition (Fig. XXIV) ; and then a vertical section will show /o?er 

 chambers, the central chamber having the undivided part (i) of the circumambient chamber 

 on one side of it, and the divided part {li, b') on the other. Each zone, as seen either by 

 transmitting light through the thinnest and most translucent specimens after mounting them in 

 Canada balsam, or by making horizontal section of thicker specimens, consists of a circular set 

 of small ovate cJiamherkfs [c, c, Fig. XXIV, and Plate IX, fig. 1), excavated, as it were, in the 



Fig. XXIV. 



Diagrammalic ix-prcseutation of the anterior of an Orbiloliies of simple type : — a, primordial chamber ; b, h, circumambient 

 chamber ; b', portion of the same partially separated by an incomplete partition ; c, c, c, chamberlets of the concentric 

 annuli ; d, passage from the primordial to the circumambient chamber ; e, e, e, passages from the cireumambieut cham- 

 ber to the chamberlets of the first annulus; /,/,/, passages from the gallery of the last anuuhis, opening at the 

 marjin of the disk. 



shelly substance of the disk, and communicating with each other laterally by annular passages 

 which unite them together into a continuous gallery. The zone which immediately surrounds 

 the " primitive disk " is connected with it by radial passages (Fig. XXI, e, e), which extend 

 from the outer margin of the large circumambient chamber to the several chamberlets of 

 which that zone is composed ; and each zone communicates with the one on its exterior by 

 similar radial passages (Plate IX, fig. 6, e, <•), which usually extend, however, not from the 

 chamberlets of the inner zone to those of the outer, but from the annular passage {li, h) of 

 the inner zone to the chamberlets of the outer ; and thus it comes to pass that the cham- 

 berlets of each zone usually alternate with those of the zones that are internal and external 

 to it. — A vertical section of the disk, such as is shown in Plate IX, fig. 2, exhibits the same 

 arrangement under a diflferent aspect. The chamberlets c, c, c of the concentric zones are 



