PLATE I. 



Structure of Calcareous Disk of Orhitolites tenuissima. 



Fig. 1. — Surface of young disk, showing its excentric spiroloculine " nucleus," giving 

 origin to successive zones of orbiculine chamberlets, whicli gradually increase in breadth 

 with the opening-out of the spire, until they extend completely round the " nucleus " ; after 

 which the successive additions are made on the cyclical plan, as concentric annuli. 

 Magnified 25 diameters. 



Fig. 2. — A portion of three peripheral annuli, enlarged to 64 diameters, and pailially 

 laid open by the removal of the superficial lamella, so as to show the two annular septa 

 aa, bb, the chamberlets c, separated by radial partitions, and the annular gallery d, into 

 which all the chamberlets open at their peripheral extremities. 



Fis. 3. — Vertical section of three annuli of the disk, taken in the radial direction, 

 so as to traverse the chamberlets lengthways; o, a, junctions of two annuli with the 

 annuli external to them ; b, h, b, annular galleries traversing the septa between the 

 chamberlets. At a, a are seen the openings through which the sarcodic cords that 

 occupy the annular galleries send radial extensions into the chamberlets of the succeeding 

 annuli. Masmified 64 diameters. 



"^tD^ 



Fig. 4. — Internal aspect of a small portion of an annulus detached Ijy fracture ; 

 showing the entrances to the chamberlets of that annulus through the septal plane. 

 Magnified 64 diameters. 



o 



Fig. 5. — External or peripheral aspect of a portion of a marginal annulus, showing 

 the passages through its septal plane, as marginal pores elongated in the plane of the 

 disk. Magnified 64 diameters. 



"o'- 



Fig. 6. — Portion of a disk, whose remainder, with the " nucleus," has been lost by 

 injury previously to the formation of the last two annuli, which have extended themselves 

 along the fractured margin, and into the nuclear space. Magnified 15 diameters. 



Fig. 7. — Incipient production of an entirely new disk, with regularly concentric 

 annuli, from a fragment of the peripheral portion of an old one. Magnified 1 5 diameters. 



