with Observations on their Internal Structure. \77 



the centre; thence to Nummulina, where there is an addition of 

 compressed chambers on each side the central plane, expanding 

 above and below into the globular form of N. ohtusa ; and elon- 

 gating in Alveolina. Returning to the subgenus of Nummulina, 

 which presents the " reticulated structure " externally, we have 

 passed on to Orbitoides, where the characteristic spiro-central 

 plane of the nautiloid forms of Poraminifera is beginning to dis- 

 appear, and then to Orbitolites, where it is entirely lost ; ending 

 with Cyclolina, which bears the same relation, in the simplicity of 

 its structure, to Orbitolites, that Operculina bears to Nummulina. 



EXPLANATION OF PLATE VIL 



Fig. 1. Operculina ineequilateralis {No. 1). 2. Vertical section of ditto. 



Fiff. 3. 0. ? (No. 2). 4. Vertical section of ditto. 



Fig. 5. Assilina irregularis (No. 1). 6. Vertical section of ditto. 



Fig. 7. A. ? (No. 2). 8. Vertical section of ditto. 



Fig. 9. Nummulina ? (No. 1). 10. Vertical section of ditto. 



Fig. 11. iV. millecaputl (No. 2). 12. Vertical section of ditto. 



Fig. 13. N. obtusa, Sowerby (No. 3). 14. Vertical section of ditto. 



Fig. 15. MeZoMJ^e5Sj?Aencfl, Lamarck (No. 1). 16. ikf. s/>Aeroic?ea, id. (No. 2). 

 17. Fascicolites eUiptica, Parkinson (No. 3). 18. Arrangement 

 of the septal lines at the apex in the last three species. 



Fig. 19. Nummulites Garausianal 20. Vertical section of ditto. 



Fig. 21. Nummularia acuta, Sowerby (No. 4): a, magnified view of reti- 

 culated structure on the surface. 22. Vertical section of ditto. 



Fig. 23. Lycophris dispansus, Sowerby (No. 1) : a, magnified view of re- 

 ticulated structure on the surface. 24. Vertical section of ditto. 

 25. Stellate arrangement of tubercles, magnified. 26. Central 

 part of central plane of chambers, magnified. 27. Portion of 

 vertical section of ditto ditto. 28. Magnified view of septa, show- 

 ing interseptal spaces and remains of interseptal vessel ? 29. Ver- 

 tical columns of cells ending in tubercles, magnified. 



Fig. 30. Orbitolites Mantelli (No. 1) : a, magnified view of reticulated 

 structure of the surface. 31. Vertical section of ditto. 32, 33, 

 34. Vertical sections of varieties. 



F^g. 35. Central plane of Orbitolites Mantelli, magnified. 36. Vertical sec- 

 tion of elongated cells of ditto. 37. Vertical section where the 

 cells are entire and have not run into each other. 38. Vertical 

 section of central part of central plane where the chambers are 

 quadrangular. 39. Ditto where the internal ends of the columns 

 interlace with each other. 



Fig. 40. Orbitolites ? (No. 2) : a, magnified view of surface, showing 



reticulated structure; b, the same still more magnified; c, ar- 

 rangement of the cells of the central plane towards the circum- 

 ference. 41. Vertical section of ditto. 

 Fig. 42. Orbitolites pedunculata (No. 3). 43. Vertical section of ditto. 



44. Arrangement and form of cells in vertical section of ditto. 



45. Ditto on the surface. 



Ann. &; Mag. N. Hist. Ser. 3. Vol. xi. 13 



