27 
this centre to every part of the circumference of the tooth, to which 
they are generally at right angles. The crown of the tooth in these 
Saurians is covered with enamel, while that part of the tooth which is 
in the alveolus is surrounded with a thick layer of cortical substance. 
In the Dolphins which have simple conical teeth like the higher 
reptiles, the crown is also covered with enamel and the base with 
cementum. But in the Cachalot and Dugong the whole of the 
teethis covered with cementum. In the Dugong this external layer 
presents the same characteristic radiated purkingian corpuscles 
or cells as in the cementum of the human teeth, and those of other 
animals ; but the cementum of the Dugong differs from that of the 
Pachyderms and Ruminants in being traversed by numerous calcige- 
rous tubes, the corpuscles or cells being scattered in the interstices 
of these tubes. Now the crowns of the teeth of the Basilosau- 
rus evidently exhibit in many parts a thin investing layer of a 
substance distinct from the body or ivory of the tooth, and the mi- 
croscopic examination of a thin layer of this substance proves it to 
possess the same characters as the cementum of the crown of the 
tooth of the Dugong. The purkingian cells are, in some places, 
scattered irregularly, but in others are arranged in parallel rows. 
The tubes radiating from the cells are wider than usual at the com- 
mencement; but soon divide and subdivide, forming rich reticula- 
tions in the interspaces, and communicating with the branches of the 
parallel larger tubes. These are placed, as in the Dugong, perpen- 
dicular to the surface of the tooth, but they are less regularly arranged 
than the calcigerous tubes of the ivory, with which, however, they 
form numerous continuations. There is a greater proportion of ce- 
mentum in the isthmus of the tooth than elsewhere; and the worn- 
down crown of the tooth must therefore have exhibited a complicated 
structure. ‘The entire substance of the ivory of the teeth consists 
of fine calcigerous tubes radiating from the centres of the two lobes, 
without any intermixture of coarser medullary tubes which charac- 
terize the teeth of the Iguanodon; or the slightest trace of the re- 
ticulated canals, which distinguish the texture of the teeth of the 
Sphyrena and its congeners. The calcigerous tubes undulate regu- 
larly, and like those of the Dugong, exhibit more plainly the pri- 
mary dichotomous bifurcations, and the subordinate lateral branches 
given off at acute angles: they also communicate with numerous 
minute cells arranged in concentric lines. 
Thus, the microscopic characters of the texture of the teeth of the 
great Basilosaurus are strictly of a mammiferous nature; and Mr. 
Owen further showed that they differ from those of the fossil Eden- 
tata, which are also surrounded by cementum, in the absence of the 
coarse central ivory ; and confirm the inference respecting the position 
of the fossil in the natural system drawn from the external aspect of 
the teeth. 
Mr. Owen then adduced further proofs of the mammiferous and 
cetaceous character of the Basilosaurus, from the structure of the 
vertebrze which proves that the epiphyseal lamine were originally 
separated from the body of the vertebrz, but were afterwards united 
