54 On the Organic and Inorganic Changes o/Parkeria. 



EXPLANATION OF PLATE V. 



N.B. — All the illustrations are more or less diagrammatic, as it would 

 be impossible in the small space of an octavo page to give them other- 

 wise ; hence this must be left for a future occasion, when it may be 

 thought desirable to draw the representations to scale in which all their 

 parts would be relatively magnified. 



Fig. L Millarella in the form of a cylindrical nucleus passing through 

 the centre of a TarJceria. Section through the centre, nat. size. 

 a, chalk-like portion charged with foraniiniferal detritus and 

 grains of glauconite, the latter represented by the dark puncta ; 

 6, point of communication with the exterior ; c, dark shades 

 intended to represent spots of Millarella scattered throughout 

 the fossil ; d, radiated structure of the Parheria. 



Fig. 2. The same, but with the Millarella of a globular form, confined to 

 the circumference of the Parheria, and thus separated from the 

 centre, which, in this instance, does not appear to possess any 

 nucleus whatever, a, Millarella; b, its communication with 

 the exterior ; c, radiate structure of the Pai-keria ; d, centre of 

 the same. 



Fi(/. 3. Parl-eria in which the central portion of the ccenenchyma has 

 become calcitized. Section through the centre, magnified two 

 diameters, a, external or petrous portion ; h and e, transforma- 

 tion of the same into calcitized tissue ; d, imaginary line of 

 separation between b and c, to point out the part where the 

 division between b and c is supposed to take place when, on the 

 cracking off of the petrous portion, c falls out in a spherical 

 form. 



Fig. 4. The same. Fragment of the calcitized ccenenchyma much mag- 

 nified, to show a a a, the " concentric lamellae," b b, the " radia- 

 ting columns," cc, the intervals between them, dd, the "tetra- 

 hedral " crystals of supposed silicate of iron. 



Fig. 5. The same. Two portions of the tubulation of the calcitized 

 ccenenchyma cut across to show their structure, greatly magni- 

 fied, a, cavity of the tubule ; b, wall of the same entire ; c, 

 external layer of the granular calcite ; d, internal layer; e e e, 

 the same, showing the remains onlg of the tubular wall ; /, cal- 

 citized tubule, in which there are no remains of the " wall " 

 whatever. 



Fig. G. The same. Lateral view of a fragment of a tubule, magnified 

 upon the same scale. «, wall of the tubule; b, its cavity ; c, 

 external layer of granular calcite ; d, internal layer of the same. 



Fig. 7. Tangential section of the central portion of a Parkeria wholly 

 transformed into Millarella, viewed from the interior or the 

 plane side. Nat. size. 



Fig. 8. The same. Fragment of the surface greatly magnified, to show 

 its reticulated structure and chalky intervals. «, veno-reticu- 

 lation, indicated by the darker shade ; b, chalk-like material in 

 which the grains of glauconite are indicated by the dark puncta. 



Fig. 9. Fi'agment of the latter very much more magnified, to show the 

 composition of the two structures, viz. : — a a, structure of the 

 veno-reticulation in the tangential section where seen " end on ; " 

 b b, filamentous parts where seen laterally ; c, chalky intervals 

 charged with foramiuiferal detritus and grains of glauconite. 



