122 J. SLADE ON PREPARING SECTIONS OF BONE AND TEETH 



cement. For microscopical examination, sections can be reduced 

 thin enough without difficulty, as the hardness, combined with 

 some amount of elasticity, renders them easily manageable. And 

 by mounting in Canada balsam, but little of the structure is lost, 

 excepting, perhaps, the lacunte and canaliculi of tbe bone cells in 

 the cement. 



Of all the fossil mammalian teeth with which I have any ex- 

 perience, that of the Megatherium is the most troublesome from 

 which to obtain fair results. The coarse vascular dentine, which 

 forms a large proportion of the tooth, crumbles away readily, even 

 after repeated macerations in fluid balsam. 



The sections, mounted dry, and sold under the name of tooth of 

 ant-eater, and showing a structure unique for mammalian tooth, 

 are sections of a molar of the Aard-vark, or Orycteropus Capensis 

 — an animal belonging to the class Edentata, common in some parts 

 of Southern Africa, and occasionally known as the Cape ant-eater. 

 The armadillo, belonging to the same order as the preceding, 

 has small, simple teeth, composed of enamel, dentine, and cement. 



For the elucidation of the ultimate structure of fish teeth, much 

 remains to be done, and a hasty glance over the plates in Owen's 

 " Odontography" and Agassiz's " Poissons Fossiles," awakens 

 the microscopist to a mine of research by no means exhausted. 



Fish teeth are of two kinds — sharp and pointed, fitted for tearing 

 prey ; and flat, broad, and rounded, close set, like pavement, fitted 

 for crushing up mollusks and crustaceans. A good example of 

 this latter is the Cestracion lying on the table. 



If we examine microscopically a number of specimens of both 

 kinds, we find them in the majority of cases to consist of dentine, 

 permeated by vessels which gradually diminish in size towards the 

 extremity of the tooth. 



In others the vessels are absent, the substance being entirely 

 dentine ; external form being no guide to internal structure. 



As examples of pointed teeth of vascular structure, we may take 

 Galeas, cod-fish, pike (recent), Lamna elegans, Oxyrrhina, Gale- 

 ocerdo, and Hemipristis (fossil). As examples of broad, crushing 

 teeth of vascular structure, we may take Myliobatis (recent), 

 iEtobatis, and Ptychodus. 



Of the non-vascular dentine, we may mention the fossil genera, 

 Sphoerodus and Lepidotus. Some there are, such as the recent 

 Lepidosteus and the fossil Dendrodus, which from the replications 



