PREFACE. XI 



they present in fossil teeth. I need only refer to the Aa^odus* the 

 8pharodus,-\ the Saurocephalus,^ the Dendrodus,^ the Lahyrinthodon,\\ 

 the Iguanodon,% and the Megatherium,^* in illustration of the value 

 of Fossil remains, and of the microscope, as an instrument in the 

 determination of their nature and affinities. 



This important application of the microscope has, however, 

 its limits, and from the difficulty of testing the described results by 

 repetition of the observations, and from other causes, it is liable to 

 be abused. 



A knowledge of the structure of the entire fossil tooth should 

 be obtained by longitudinal and transverse sections ; at least by a 

 transverse section through the entire thickness of the crown. With 

 this knowledge a fragment of the tooth of the same species may 

 afterwards be determined : and also in many cases those of other 

 species of the same genus, or natural family : without it, great 

 mistakes may be committed. If, for example, a microscopic observer 

 had begun his examination of an Iguanodon's tooth by a slice of a 

 fragment from the outer half of the crown, and had afterwards 

 examined another fragment of the tooth of the same species taken from 

 the inner half, he would, most probably have referred such fragments 

 to two very distinct species of animals. The requisite knowledge of 

 the characteristic combination of one lateral moiety of dentine, and 

 another of vaso-dentine in the same tooth, pre-supposes the examina- 

 tion of a section of an entire specimen. In like manner, to pro- 

 nounce on the generic and specific distinctions of fossil Proboscidians 

 from the characters of portions chipped off the exterior of their tusks, 

 is an abuse of the microscope, and betrays an ignorance of the mode 

 and limits of its application. ff 



One consequence of an attempt, like the present, to determine 



* PI. 14. t PL 32. X PI. 55. 



§ PL 62 B. The teeth of this extinct Fish afford a beautiful example of the unexpected 

 application of microscopic characters of dental tissue in the determination of an important 

 geological problem. — See Appendix to Mr. Murchison's ' Geology of Russia/ p. 635. 



II PI. 64 A. IT PL n. ** PL 84. 



ft Thus, out of portions of tusks of young and old individuals of the Mastodon giganteits, 

 the genera Missourium, Tetracaulodon, and their different species, have been attempted to be 

 established. — See Geological Proceedings, June 29, 1842. 



