78 
form can be identified only with difficulty. — Abequt the origin of 
these transformations we know very little, as may be said about 
the derivation of the most primitive (Triassic) tooth-ferms from 
the reptilian teeth or about their connection with the Eocen& forms; 
that a mammalian tooth has its descent from a reptilian ane is 
probable, but not proved. 
In the same year the Concrescence Theory reaches its full deven 
lopment through Kikenthal's "Ursprung und Entwickelung der 
Såugetierzåhne.” The dentition of the whalebone whales, usually 
looked upon as primitive, can hardly be so, the hindmost teeth of 
the embryos being rounded, even tricuspid, so that the homodonty 
must be secondarily acquired. Geoffroy St. Hilaire and more espe- 
cially Eschricht together with Julin and Weber have shown us the 
rudimentary teeth of the Mystacoceti; some of them are double, 
others show irregular cusplets which by Julin and Weber are inter- 
preted as rudiments of an earlier multicuspid condition. But this 
cannot be so, as the cusplets are only evidence of a beginning 
resorption. The compound teeth-rudiments on the contrary are 
evidences of an earlier condition, since, as can be seen in a series 
of foetal jaws of the same species, the number of the tooth- 
cusps up to a certain time is constant (in Balænoptera musculus 53), 
whereas the number of compound teeth (2—3 cusps joined together) 
decreases during growth, Only when all the compound teeth, with 
the exception of two or three, are separated, does.their resorption 
begin, and the number of cusps decreases, From this we see that 
the compound tooth is of a primitive character, and that this tooth 
by secondary prolongation of the jaws can be resolved into teeth 
with a single cusp. Therefore it may be supposed that the com- 
plex molars really derive their origin from a concrescence of uni- 
Cuspid tonical teeth. Evidence of this hypothesis we" have fi) 
in the teeth of Triconodon, where three homogenous eusps are 
placed. in one løngitudinal row. In the seals also we find isolated 
examples of the division of one tooth into two. The evolution of 
the -mammalian teeth may be supposed. to be the following: The 
