26 
a form of the crown similar to that of the Basilosaurus, but the 
opposite indentations are not so deep; and the entire grinding sur- 
face of the molars of the Manatee differs considerably from those of 
the Basilosaurus, the anterior supporting two transverse conical 
ridges, and the posterior three. The Dugong resembles more nearly 
the fossil in its molar teeth; the anterior ones being smaller and 
simpler than the posterior, and the complication of the latter being 
due to exactly the same kind of modification as in the Basilosaurus, 
viz. a transverse constriction of the crown. ‘The posterior molar 
has its longitudinal diameter increased, and its transverse section 
approaches to the hour-glass figure, produced by opposite grooves. 
There is in this tooth also a tendency to the formation of a double 
fang, and the establishment of two centres of radiation for the calci- 
gerous tubes of the ivory, but the double fang is probably never com- 
pleted. The teeth in the Dugong moreover are not scattered as in 
the Basilosaurus. 
Mr. Owen then briefly compared the teeth of the fossil with 
those of the Saurians, and stated that he had not found a single 
instance of agreement in the Basilosaurus with the known dental 
peculiarities of that class. From the Mosasaurus the teeth of 
the American fossil differ in being implanted freely in sockets and 
not anchylosed to the substance of the jaw ; from the Ichthyosaurus 
and all the lacertine Sauria in being implanted in distinct sockets, and 
not in a continuous groove; from the Plesiosaurus and crocodilian 
reptiles from the fangs not being simple and expanding as they de- 
scend, but double, diminishing in size as they sink in the socket, 
and becoming consolidated by the progressive deposition of dental 
substance from temporary pulp in progress of absorption. In the 
Enaliosauria and the Crocodilia, moreover, there are invariably two 
or more germs of new teeth in different stages of formation close to 
or contained within the cavity of the base of the protruded teeth ; but 
the Basilosaurus presents no trace of this characteristic Saurian 
structure. From the external characters only of the teeth, Mr. Owen 
therefore infers, that the fossil was a Mammifer of the cetaceous 
order, and intermediate to the herbivorous and piscivorous sections 
of that order, as it now stands in the Cuvierian system. 
In consequence however of the Basilosaurus having been re- 
garded as affording an exceptional example among reptilia of teeth 
having two fangs, though contrary to all analogy, and as the other 
characters stated above, may be considered by the same anatomists 
to be only exceptions, Mr. Owen procured sections of the teeth for 
microscopic examination of their intimate structure and for com- 
paring it with that of the teeth of other animals. 
In the Sphyreena and allied fossil fishes which are implanted in 
sockets, the teeth are characterised by a continuation of medullary 
canals, arranged in a beautifully reticulated manner, extending 
through the entire substance of the tooth, and affordimg innumerable 
centres of radiation to extremely fine calcigerous tubes. 
In the Ichthyosaurus and Crocodile the pulp cavity is simple and 
central, as in Mammalia, and the calcigerous tubuli radiate from 
