284 On the Dentition of Mammals. 
The following conclusions result from the foregoing investi- 
gations into the dentitions of mammals. The rudiments of 
both dentitions occur not only in the higher mammals, but 
also in the lower orders of Marsupials, Edentates, Odontocetes, 
and Mystacocetes. ‘THE EARLIEST MAMMALS WERE DIPHYO- 
pont. ‘The monophyodont and homodont condition of many 
mammals, e. g. the Toothed Whales, has been secondarily 
acquired. Within the mammalian class, ascending from the 
lowest to the highest forms, we see how the secon dentition 
gains the upper hand more and more as regards form and 
function, while in the lower forms the first dentition is pre- 
dominant. In the rudimentary stage both dentitions are of 
equal value ; embryology gives us no support for the often- 
expressed assertion that one of the two dental rudiments has 
arisen in dependence upon the other; they are both sisters, 
whose mother is the simple invagination in the jaw, which we 
term the dental fold (‘ Zahnleiste’). 
Now can we discover a bridge which connects the dentition 
of Mammals with that of their ancestors, the Reptiles ? 
There are no absolute differences between the mammalian 
and reptilian tooth, as has already been shown by Seeley * ; 
not one of the characters of the mammalian tooth is perfectly 
constant ; the loss of any one of them is an approximation to 
the reptilian tooth, and conversely reptilian teeth often 
assumed characters belonging to those of mammals. The 
replacement of teeth moreover occurs in reptiles to a still 
greater extent than in mammals, since several series of teeth 
may follow one another, the rudiments of which, as in the 
case of the second dentition of mammals, are formed inter- 
nally to the first. The idea of deriving the dentition of 
mammals from that of reptiles therefore does not appear to 
me to be too hazardous ; of the several series of teeth which 
are found in reptiles, only two still persist in mammals, 
In conclusion I would subjoin the following attempt to 
explain the origin of molar teeth in mammals, while freely 
admitting its purely hypothetical nature. Owing to our 
investigation of tooth-germs in Whalebone Whiales, we have 
become acquainted with the phenomenon of the division of 
the molars in mammals, whose jaws become elongated, into a 
multitude of conically pointed structures, resembling the teeth 
of reptiles. Conversely, have not the ‘molars of | mammals 
also arisen in this way, in that, in consequence of the reverse 
process, a shortening of the jaws, which the ancestors of 
existing mammals underwent in the course of their trans- 
* H. G. Seeley, “On the Nature and Limits of Reptilian Character in 
Mammalian Teeth,” Proc, Roy. Soc, Lond. vol. xliy. pp. 129-141. 
