SHARKS. 



33. 



tooth : they ultimately terminate in flattened sinuses, which anasto- 

 mose and form the boundary line between the central osseous and 

 the dense exterior enamel-like substance of the tooth. The whole of 

 the superficial part of the tooth is occupied by minute calcigerous 

 tubes,(l) which proceed in a wavy course, generally at right angles to 

 the external surface ; they ramify at very acute angles, and their 

 terminal branches anastomose, and most of them end in a 

 series of calcigerous cells, situated beneath 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. The medullary 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 ; and 

 have a more irregular wavy course than the superficial calcigerous 

 tubes. They form, by their numerous anastomoses an inextricable 

 reticulation, (2) and their terminal ramuU dilate into or communicate 

 with calcigerous cells. 



The medullary canals of the teeth are occupied in the recent 

 dog-fish by a sanguineous medulla, closely resembUng that which 

 fills the medullary cells of the coarse bone of which the base of the 

 tooth is composed, and with which cells the anastomosing reticulate 

 canals of the crown of the tooth are directly continuous. In the old 

 exterior teeth a great proportion of the medullary canals are con- 

 solidated by concentric layers of earthy deposit. 



In the fully formed flat teeth of the foetal Carcharodon, which 

 were sufficiently transparent to render their texture perfectly discern- 

 ible by transmitted Hght, this was seen to be essentially the same as 

 in the Lamna ; but the disposition of the medullary canals was more 

 regular : the median branches were continued as in Lamna to the 

 apex of the tooth parallel with the axis, the lateral ones gradually 

 inclined to the sides of the tooth, their direction being more trans- 

 verse as they approached the base ; and here inclining downwards to 

 the bilobed osseous root. The branches of the medullary canals are 



(1) A highly magnified view of the calcigerous tubes continued from the peripheral medul- 

 lary tube h is given in PI. 7. 



(2) See the view of these tubes between the medullary canals a and 6, PI. 7. 



D 



