TBANSACTIONS OF THE SECTIONS. 139 



much branched; but towards the periphery of the tooth, the ramifiea 

 tubes are all directed, as in true ivory, at right angles to the superfi- 

 cies, and thus constitute a regular layer of calcigerous tubes, disposed 

 so as to offer the greatest resistance to pressure. This layer is equal 

 in thickness to about one-fifteenth part of the vertical diameter of the 

 thickest part of the tooth. 



The finest or terminal branches of this peripheral layer of tubes, I 

 have traced in various places into what at first sight appears to be the 

 enamel. Under a magnifying power of 400 diameters, however, this out- 

 ermost layer is seen to be composed of extremely minute tubes, xoV)o ^-^ ^^ 

 a line in diameter'; they are branched like the coarser tubes of the body of 

 the tooth; irregularly wavy in their course; having a general tendency 

 to an arrangement at right angles to the superficies, but inextricably 

 interwoven, and connected anastomotically together, so as to require a 

 strong light to penetrate even the thinnest section, and render their 

 structure and an'angement visible. The continuation of these finer 

 superficial tubes, witli the coarser tubes of the body of the tooth, is best 

 observed by changing the focus, which brings the transitional tubes at 

 different depths in the section into view. In some parts of the section, 

 a medullary or Haversian canal is dis23layed longitudinally ; and the 

 parallel lines of the surrounding concentric strata on each side are ex- 

 hibited. The canal maintains a general uniform diameter, but slightly 

 dilates where it divides or sends off a cross branch to communicate 

 with the adjoining canals. These canals commence from the large cells 

 of the bone of the base, and pass into the substance of the tooth to- 

 wards its periphery ; communicate by transverse canals, but all ulti- 

 mately terminate in bundles of the wavy ramified calcigerous tubes of 

 the body of the tooth. I conclude that the coarser canals were occu- 

 pied by a vascular pulp in the living animal, and that the fine terminal 

 tubes were the seat of the salts of lime. The silex occupying the lon- 

 gitudinal canals and coarser tubes, has received a dark stain, probably 

 from the colouring matter of the vascular pulp, — but the finer tubes, 

 from the want of this difference of colour, are in many parts obscurely 

 visible, if at all. They are disceiniible in some situations crossing the 

 concentric lamellae at right angles to the central canal. The chief dif- 

 ference between the appearance presented by the Haversian canals of 

 the tooth of Acrodus, and those in true bone, is in the absence of the 

 cells or corpuscles. These are apparent only at the base of the tooth — 

 irregular in size and form, very minute, and appearing like simple gra- 

 nules without radiating lines. The character of the main or coarser 

 canals and calcigerous tubes of the ivory of the tooth of Acrodus, re- 

 poses on their undulating course, their rapid diminution and bi'anching, 

 and the moderately acute angles at which the branches are given off, 

 except at the circumference of the tooth, where they run nearly parallel 

 to each other. In other parts they closely resemble the branching of 

 trees. The line of demarcation between the coarser and finer ivory is 

 formed by a series of small cells of a similar granular appearance to 

 those at the base, in which many of the finer branches of the coarse 

 ivory terminate, and from which the minute tubes of the enamel-like 



