112 OS THE MICEOSCOPIC STEFCTUBE OF THE 



But before entering into tlie microscopical structure I wish to 

 quote Dr. Giinther's remarks on a vertical section of a tootli of 

 Ceratodus, as tlie characters he describes are identical with those 

 of a tooth of C. tuhfi)'culatus, G. cristatus, <fcc. " In a vertical 

 section of one of the grinders it is seen that the real depth of the 

 tooth (that is, that portion which is formed by dentine) is much 

 less than it appears from a merely outward inspection. It rests, 

 in fact, on an elevated plateau of the dentary bone, which has 

 exactly the same outlines as the tooth itself, and the substance of 

 which passes so gradually into that of the tooth that it is only 

 by the difference in the shade of colour that the boundary between 

 osseous base and dentinal crown is indicated. This anchylosis, 

 however, is Hmited to the circumference of the base of the tooth ; 

 for its central parts are separated from the bone by the extensive 

 but shallow pulp-cavity. We must remember that our specimens 

 of living Ceratodus are by no means aged individuals, certainly 

 much smaller and younger than those gigantic individuals of 

 extinct species must have been, of which teeth two and more inches 

 long are preserved. In such fossil teeth no pulp-cavity is visible, 

 but the dentine passes into the bone across the whole base of the 

 tooth. It is not at all improbable that the pulp-cavity disappears 

 altogether with age." The last remark concerning there not 

 being any pulp-cavity in the teeth of fossil Ceratodi certainly does 

 not apply to the teeth of Ctenodi, for I have examined vertical 

 sections of teeth of C. tuherculatiis that displayed just as distinct 

 pulp-cavities as we see in similar sections of the teeth of Cera- 

 todus Forsteri, and others again that exhibited no trace of such 

 spaces. Dr. Giinther's hint as to the presence or absence of a 

 pulp-cavity being an indication of age is, in my opinion, 

 undoubtedly true when applied to the teeth of Ctenodus. 



A vertical section of C. tuhei^culatus taken either from before 

 backwards, or from side to side, when slightly magnified, shows 

 that the osseous tissue of the tooth is exceedingly freely permeated 

 by medullary canals, which are very large in diameter, and which 

 branch and anastomose with each other frequently but yet in a 

 very irregular manner, so that the tissue presents an appearance 

 of a network of vessels, the meshes of which vary much 

 in size. Though this is the aspect observed in a brief glance. 

 a closer inspection makes evident that there is a tendency 

 of the canals towards a certain course, and towards a regular 

 method of branching also, in some parts of the tooth ; for the 

 arrangement of these tubes varies according as to whether we 

 examine the base, or plate as it is usually termed, or the ridges 

 with their tubercles. We have in these two parts fundamental 

 differences of structure, apart from the characters of the canals, 

 the plate possessing certain features found in true bone structure, 

 while the ridges present characteristics that are purely dental. 



