17G DENDRODUS. 



its base to its sub-acute apex. The surface of the tooth is much 

 smoother than in the Dendrodus strigatus, I took a transverse section 

 from the base of this tooth, and finding the same essential radiated 

 disposition and dendritic ramifications of the medullary canals, I 

 made a longitudinal section of the rest of the tooth. 



In this section eight principal medullary tubes were seen ; the 

 four largest and closest together were situated in the centre ; the others 

 diminishing as they approached the side. The central canals run pa- 

 rallel to each other and to the axis of the tooth along the basal half ; 

 the two middle canals so continuing to the apex. The small lateral 

 tubes bend outwards to the side of the tooth ; the third on each side 

 sends off one or two primary branches in the same obhque direc- 

 tion and also finally terminates by bending towards the margin of the 

 tooth. The primary divisions of the large medullary tubes run parallel, 

 or nearly so, to the trunks, but begin slightly to diverge as they 

 approach the apex. The primary divisions of the small lateral tubes 

 are given off at different, but always acute angles. The smaller or 

 secondary branches are given off" frequently at right angles, and those 

 from the primary branches sometimes retrograde to the main tubes. 

 The plan of branching of the whole system of medullary canals which 

 pervades the entire substance of the tooth is strikingly similar to that 

 of certain forest trees, as the elm, but with the characteristic diffe- 

 rence, which distinguishes in this, as in most other cases, the branching 

 of an animal from a vegetable structure, namely, that the terminal 

 branches anastomose, forming a coarse but elegant net-work, 

 occupying with considerable regularity the whole of the interspaces 

 of the longitudinal medullary tubes. 



A transverse section of this tooth exhibits three-fourths of its 

 central part occupied by a reticulo-meduUary structure ; and the 

 peripheral portion traversed by medullary rays. In the central 

 structure are seen the arese of numerous, nearly equal sized and 

 equi- distant medullary canals, around which the dentine is disposed 

 in well-marked concentric layers, and from which the calcigerous 

 tubes radiate as in the Myliobate and Orycterope ; but these tubes 

 are relatively fewer and less parallel in their course, and form more 

 decided reticulations in that course. The shape of each component 



