IN PALEONTOLOGY. 169 



bably originally siliceous, but is now iron pyrites. Owing, appa- 

 rently, to some difference in the refractive index of colloidal 

 and crystalline silica ; fossil siliceous fibres and spicules, 

 mount much better in Glycerine jelly than in Canada balsam. 

 Recent spicules, on the other hand, are invisible in Glycerine 

 jelly, but the fibre is more than usually well defined. Recent cal- 

 careous spicules polarise well, but siliceous spicules do not. 



Foraminifera and Polycystina and many other orders are almost 

 entirely microscopic, and are too well known to need any notice, 

 however brief. Fossil botany, also, is an extensive subject, and 

 one in which the knowledge of microscopic structure is all-import- 

 ant, and roots no longer do duty for branches, and rootlets for 

 leaves, as they did formerly, when external appearance was taken 

 as the main, if not the only guide. 



In conclusion, I believe that you will find that the use of the 

 microscope in Palaeontology greatly aids in drawing these conclu- 

 sions ; that though time and external appearances may widely sepa- 

 rate various beings, yet that they all bear definite relations to each 

 other, and follow the same laws ; that the life-history of the indi- 

 vidual, from its earliest stage to adult perfection, runs parallel with 

 the life-history of the race, and that as the pedigree of many 

 existing beings can be roughly traced in the annals of the past, 

 what is true of a part will ultimately be found to be true of the 

 whole, so that the old and the new are not really separate, but 

 form a portion of a wondrous and complex whole, which, although 

 for ever slowly changing, seems for ever to lead to greater com- 

 plexity and beauty. 



EXPLANATION OF PLATE XVII. 



Fig. 1.— Section of upper jaw and teeth of the Megatherium, one-third 

 natural size (after Owen). The teeth are five in number 

 on each side of the upper jaw, as drawn, and four on each 

 side of the lower jaw. p., the pulp-cavity, which is unusually 

 extensive, and from the apex of which a fissure is continued 

 to the middle depression of the grinding surface of the tooth. 

 t. , the vaso-dentine, the central axis of which is surrounded 

 by a thin layer of hard or unvascular dentine, d. , and this is 

 coated by the cement, c. , which is of great thickness on the 

 anterior and posterior surfaces, but thin where it covers the 

 outer and inner sides of the tooth. 



