GENUS ORBITOLITES: EXPLANATION OF PLATES. 233 



type ; from b to c the annular canal double, and an irregular intermediate 

 stratum interposed, from which, however, the superficial cells are not com- 

 pletely differentiated ; and from c to d the imperfect separation of the super- 

 ficial cells from the intermediate layer, and the extreme irregularity of the 

 latter: 50 diam. 



Fig. 8. Tangential section, near the margin of the same disk, showing the same 

 peculiarities : 50 diam. 



Fig. 9. Central portion of a disk, showing the nuclear cavity divided into two (the 

 section having traversed the central cell and one side of the circumambient) ; 

 the formation of the first three zones (a b) on the simple type ; the im- 

 perfect separation of the superficial cells from the intermediate stratum in 

 the next two zones (b c) ; and the regular development of the remainder 

 (c d) on the complex type : 50 diam. 



Fig. 10. Vertical section of the central portion of a disk of complex type; showing 

 four cavities in the nucleus ; the first five zones (a 6) constructed upon 

 the simple type, with a single annular canal, but an incipient separation 

 manifesting itself between the superficial cells and the intermediate stratum ; 

 in the next three zones (b c) the annular canal double, but the superficial 

 cells still partly continuous with the columnar ; the outer part (c d) 

 framed according to the regular complex type: 48 diam. 



Fig. 1 1. Vertical section of a disk of complex type, showing an unusual development 

 of vertically superposed cells immediately around, and even partly cover- 

 ing, the nucleus, from which they arise by four layers of passages ; a pro- 

 gressive diminution in the thickness of the disk, as far as the fifth zone (b), 

 with a gradual approximation towards the regular type, on which the 

 remainder (b c) is developed : 48 diam. 



Fig. 12. Vertical section of a disk of complex type, showing commencement of its first 

 zone by a single layer of passages (a) from the nucleus ; the presence of 

 two annular canals, with the absence of separation of the superficial cells, 

 in the first zone (b) ; the incomplete separation of the superficial cells from 

 the intermediate columnar portion in the second zone (c) ; and the normal 

 conformation of the remainder (c d) : 48 diam. 



PLATE VI. 



Fig. I. Portion of a disk of simple type, from the immediate neighbourhood of the 

 nucleus, showing the beak-like projections at the sides of the passages very 

 remarkably developed: 100 diam. 



Fig. 2. Horizontal section of the intermediate layer, passing (in consequence of the 

 flexure of the disk) through two different planes, and thus showing the 

 two different directions of the oblique passages which connect the colum- 



