114; OX THE MTOEOSCOPIC STEUCXrEE OF THE 



canals, tliat is, it increases in quantity, but tte addition is never 

 in great excess, the sections, whether vertical or transvei*se, 

 always having an open structure. The bone substance still 

 remains homogeneous, but it often presents a laminated appear- 

 auce immediately surrounding the canals ; lacunte are to be 

 seen, but they are much smaller, more rounded, and few and far 

 between. 



On a level with the bases of the ridges, the network character 

 of the medullary system is quite lost, and the structure of the 

 ridges and tubercles presents a totally different arrangement 

 from that of the base; when a low magnifying power is employed 

 for observation the change seems to be sudden. The canals are 

 now much decreased in diameter, run in a somewhat vertical 

 manner, give off branches at a very acute angle, which anastomose 

 with each other very freely after pursuing a short vertical and 

 oblique course. From these causes, a section when examined 

 appears filled with very short vertical tubes, as is portrayed in 

 Pig. X., which is a vertical section of a tubercle of C. tiiberculatus 

 magnified 20 diameters. The osseous substance immediately 

 surrounding the medullary canals is laminated, and the concentric 

 rings are darker in tint than the unlaminated portion of the 

 tissue ; the whole, however, is homogeneous, and the lacunae have 

 disappeared. From the canals spring numerous dentinal tubules, 

 which run a very short course and branch once or twice on their 

 way in a dichotomous manner. The tubules of one canal 

 inosculate by their terminal branches with those of a neighbour- 

 ing canal, and they also undoubtedly anastomose with the 

 tubules adjoining that arise from the same main vessel. These 

 tubules are often not visible in sections, sometimes from the 

 Canada balsam, which has often the same degree of refrangibility 

 as the fossil substance, permeating them, but more frequently 

 they have been ground away in attempting to make a thin 

 section. Fig. XL represents a portion of a medullary canal cut 

 vertically, and shows the laminated character of the osseous tissue 

 adjoining, and the tubules springing from the canal as seen 

 under a magnifying power of 250 diameters. The structure 

 I have just described is present throughout the ridges and 

 tubercles. 



Immediately external to this tubular portion of the ridges 

 and tubercles on the upper surface of the tooth is a thin layer of 

 dense tissue, unpermeated by the medullary canals or their tubuli, 

 and apparently without structure. When an unworn tooth is 

 examined there is perceived external to the above dense layer a 

 coat of ganoine or fish enamel, which is also structureless ; this 

 covering, however, is rarely seen, for it appears to have been very 

 easily worn away by the friction which ti'ituration of liard sub- 

 stances with teeth like these would cause. 



