TRANSACTIONS OF THE SECTIONS. 141 



viewed with the naltecl eye, to be composed of a close-set series of pa- 

 rallel coarse tubes, dividing dichotoniously, and united together here 

 and tliere by short transverse arches with the convexity towards the 

 grinding surface. The diameter of the interspaces of these canals is 

 generally equal to between two and three diameters of their areae. 



Viewed by a higher power, the tubes are seen to be innuediately 

 surrounded by a clear amber-coloured substance, analogous to that 

 which forms the concentric layers around the canals, which I have 

 compared to those described by Havers in true bone. 



Under a power of 400, the large canals ai-e seen to send off from 

 every part of their course numerous minute tubes, generally at right 

 angles, to the medullary canals ; these tubes run irregularly, ramify, 

 and anastomose in the interspaces of the medullary canals, and form a 

 coarse matting or plexus of tubes, the number of which sometimes 

 quite intercepts the light. 



In the teeth of the genus Lamna, a number of medullary canals are 

 continued from the short and small pulp-cavity at the base of the tooth, 

 which ramify and anastomose, so as to form a beautiful reticulate 

 arrangement of tubes, very similar to a network of capillary vessels, 

 throughout the whole substance of the tooth : they ultimately termi- 

 nate in a flattened sinus, which seems to extend over the whole tooth 

 at a veiy short distance from its superficies. The whole of the super- 

 ficial part of the tooth is occupied by minute calcigerous tubes, which 

 proceed in a wavy course, generally at right angles to the external 

 surface ; they ramify, and their terminal branches anastomose, and 

 many of them terminate in a stratum of calcigerous cells, situated be- 

 tween the body of the tooth and what appears to be the outer stratum 

 of enamel. In this stratum, however, there are evident traces of a 

 series of much finer tubes, continued from the preceding layer of cells, 

 which proves that this is not true enamel, but a fine kind of ivory, like 

 that in the tooth of the sloth and megatherium. The coarse reticulate 

 canals in the body of the tooth are surrounded by concentric layers, 

 traversed by the calcigerous tubes which are everywhere given off at 

 right angles from the larger canals ; these canals are occupied, in the 

 recent fish, bj' a sanguineous medulla, closely resembling that which 

 fills the medullary cells of the coarse bone, to v/hich the base of the 

 tooth is anchylosed, and with which cells the anastomosing reticulate 

 canals of the tooth are directly continuous. 



Carcharias Megalodon. — The calcigerous tubes at the superficies of 

 this tooth are disposed in groups which, M'ith an insufficient magnifying 

 power, appear like single coarse tubes, but with a higher power, are 

 seen to be composed of congeries of parallel tubes, apparently twisted 

 together. The interspaces are nearly equal to the diameter of these 

 curious fasciculi : they are occupied by more scattered tubes, and by 

 short oblique or transverse anastomosing branches. At one part of a 

 section of this tooth, the peripheral coarse sinus or canal, which always 

 runs parallel with the superficies, gave off an infinite number of minute 

 tubes, which formed a plexus, (or plexiform stratum,) and from the 

 outer part of this plexus, the tubes above described passed, at right 



