460 On the Flint Nodules of the Trimmingham Chalk. 



and cavities. And above and beyond the particular question 

 of the formation of flint is the general fact that of the vast 

 multitude of spicules which must have existed in nearly all 

 stratified formations, only an insignificant remainder is now to 

 be found ; and in those which have disappeared we have the 

 key to the great variety of silicifications which characterize 

 ancient sediments. 



EXPLANATION OF THE PLATES. 



Plate XIX. 



Figs. 1-3. Spicules of Discoder mites cretaceus, Soil. 



Figs. 4, 5. Corallistes cretaceus, Soil. 



Fig. 6. Curved acuate spicule, possibly from an Echinonematous sponge. 



Fig. 7. Cylindrical spicule, possibly from Corallistes cretaceus. 



Fig. 8. Rhagadinia Zitteli, Soil. : dermal spicule. 



Fig. 9. Acuate of C. cretaceus? 



Fig. 10. Rhagadinia Zitteli: body-spicule. 



Fig. 11. Lithistid spicule. 



Fig. 12. C. cretaceus (?) : body-spicule. 



Fig. 13. Kanodiscites parvus, Soil. : dermal spicule. 



Fig. 14. Furydiscites irregularis, Soil. : dermal spicule. 



Fig. 15. Small acerate (R. Zitteli?). 



Fig. 16. Macandrewites Vicargi, Carter : body-spicule ? 



Fig. 17. Tuberculated skeleton-corpuscle. 



Fig. 18. Podapsis cretacea, Soil. : body-spicule. 



Fig. 19. Forked spicide (P. cretacea?). 



Fig. 20. Macandrewites Vicargi, Carter : dermal spicule. 



Figs. 21, 22. Compsapsis cretacea. Soil. : body-spicule. 



Fig. 23. Podapsis cretacea. 



Fig. 24. Disciform spicule of unknown nature. 



Figs. 25, 26. Podapsis parva, Soil. : body-spicules. 



Fig. 27. Dermal spicule (Corallistes ?). 



(All magnified 54 diameters.) 



Plate XX. 



Figs. 28, 29. Pachastrellites fmifer, Soil. 



Fig. 30. Pachastrellites (? globiger). 



Pigs. 31, 32. Tethylites cretaceus, Soil. 



Fig. 33. Acuate spicule of T. cretaceus (?). 



Figs. 34, 35. Geodites cretaceus, Soil. 



Fig. 36. Geodites ; small globate spicule. 



Fig. 37. G. cretaceus. 



Figs. 38, 39. Pachastrellites globiger, Soil. 



Fig. 40. P. globiger (?). 



Fig. 41. Dercitites haldonensis, Carter (?). 



Fig. 42. Triphgllactis elegans, Soil. 



Fig. 43. Geodites cretaceus. 



Fig. 44. Pachcena Hindi, Soil. 



F'ig. 45. Geodites cretaceus. 



Fig. 46. Rhopaloconus tuber culatus, Soil. 



Fig. 47. Dercitites haldonensis ? 



Figs. 48, 49, 50. Casts of Foraminifera. 



Fig. 51. Head of an acuate spicule (? Tethylites). 



Fig. 52. Pachcena Hindi. 



