GENUS ORBITOLITES: EXPLANATION OF PLATES. 231 



Fig. 6. Nucleus and first two annular zones of another disk, showing a deficiency in 

 the connexions of the first annular zone with the circumambient segment bb, 

 for about a third of the circumference of the latter, between the points 

 cc: 84 diam. 



Fig. 7- Portion of one of the surfaces of an animal of complex type (as in fig. 4) : 

 aa, aa, rows of superficial cells, connected at their two extremities with 

 the annular bands bb, bb: 150 diam. 



Fig. 8. Portion of fig. 1, enlarged, to show the ordinary mode in which the segments 

 of each zone are connected by peduncles with the annular band of the pre- 

 ceding zone, so as to alternate with its segments ; showing also the 

 occasional interpolation of additional segments, a, a, whose peduncles come 

 off from the segments of the preceding zone: 90 diam. 



Fig. 9. Portion of a section of the shelly disk enlarged, to show the corresponding 

 appearances it presents ; each cell being ordinarily connected by a radial 

 passage with the annular canal of the preceding zone, and thus with the 

 two cells alternating with itself; but cells a, a, being sometimes interpo- 

 lated, which open directly into the cells of the preceding zone : 90 diam. 



Fig. 10. Central and circumambient segments (a and bb) of a large disk, showing 

 the origin of the segments of the first annular zone, c, c, from less than half 

 the circumference of the nucleus : 84 diam. 



Fig. 11. Peculiar bodies (ova?) found in the substance of the sarcode in different 

 parts, showing successive stages, a, b, c, d, e, f, of binary subdivision ; 

 g, other bodies of somewhat larger size, found in one of the superficial cells 

 of a vertical section : 130 diam. 



Fig. 12. Central portion of fig. 1, enlarged to the same scale as figs. 5, 6, 10 ; show- 

 ing the central segment a, the circumambient segment bb, and the origin 

 of the first annular zone in three peduncles proceeding from the end of the 

 latter : 84 dfam. 



Fig. 13. Central portion of another disk, showing the origin of the first annular zone 

 by eight peduncles from the circumambient cell : 84 diam. 



PLATE V. 



Structure of the Calcareous Disks of Orbitolite. 



Fig. 1. Ideal Representation of a Disk of the Simple Type, the details of the different 

 parts rnade-up from actual specimens ; showing the natural surface, with 

 the markings of the cells ; the natural margin, with the single row of pores 

 between the protuberances of the cells ; a portion of the interior, as dis- 

 played by a horizontal section, showing the central cell a, the circumam- 

 bient cell b, b, the concentric zones of cells c, c, with the annular passages 

 which connect together the cells of the same zone, and the radiating passages 



2 H 2 



