74 Mr. Owen on the Zeuglodon Cetoides. 



central, as in the Mammalia. The calcigerous tubuli radiate from this centre to 

 every pa t of the circumference of the tooth, to which they are generally placed 

 at right angles*. The whole of the ' dentine,' or ivory of the tooth, is composed 

 of these minute tubules and their connecting tissue ; but that part of the dentine 

 which forms the crown is covered with enamel, while that part which is placed 

 in the alveolus is surrounded with a thick layer of cortical substance, filling up 

 the grooves of the fluted base of the tooth. 



In the Dolphins, which have simple conical teeth like the higher reptiles, the 

 inserted base is likewise covered with csementum, but the crown with enamel. 



In the teeth of the Cachalot there is no true enamel, but the whole exterior is 

 invested with a thick layer of ccementum, which is sometimes as thick as the in- 

 cluded portion of true tubular ivory. 



In the Dugong, also, both the exposed and inserted parts of the molar teeth are 

 coated with csementum, and this substitute for enamel presents the same charac- 

 teristic radiated corpuscles or cells as were discovered by Purkinje in the csementum 

 of the human teeth, and of other animals ; but the cajmentum of the Dugong differs 

 J'rom that of the Pachyderms and Ruminants in being traversed by numerous 

 fine tubes, pretty closely aggregated, and giving off numerous branches, the cor- 

 puscles or cells being scattered in the interspaces of the tubes, which here and 

 there communicate with the true calcigerous tubes of the ivory. Now the crowns 

 of the teeth of the Zeuglodon evidently exhibit in many parts an investment of a 

 thin layer of a substance distinct from the body or ivory of the tooth, the real 

 nature of which investment microscopic examination could alone determine. 



In a fine section of this part of the tooth of the Basilosaurus, taken from about the 

 middle of the exposed crown, I find that this investment is csementum and not ena- 

 mel, and that it presents the same microscopic characters as the csement of the crown 

 of the teeth of the Dugong. The Purkinjean cells are scattered in some places 

 irregularly, in others arranged in parallel rows ; they are about ^yTjth of a line in 

 diameter, generally of an oval form, but with very irregular outlines : the tubes 

 radiating from the cells are wider than usual at their commencement, but soon 

 divide and subdivide, forming rich reticulations on the interspaces, and com- 

 municating with the branches of the parallel larger tubes. These are placed, as 

 in the Dugong, perpendicularly to the superficies of the tooth, but are less regu- 

 larly arranged than the calcigerous tubes of the ivory, with which, however, they 

 form numerous continuations. There is a greater proportion of the ceementum at 

 the isthmus of the tooth than elsewhere. 



The entire substance of the ivory of the tooth consists of fine calcigerous tubes, 



* The general direction of the calcigerous tubes is indicateJ by the fine white lines in figs 1 and 2. 

 PI. VIII. 



