1893.] Succession of the Teeth in Mammals. 501 
observed in the whalebone whale, we also find in young 
embryos of the Edentatesa double pointed or biconodont tooth, 
which later divides secondarily into its single pointed compo- 
nents. The author considers it doubtful whether this bicono- 
dont type is primitive, or as seems to him more probable, is 
derived by degeneration from a triconodont type ! 
As regards the bearing of paleontology and comparative 
anatomy upon these facts, we find rudimentary incisors in the 
living Dasypus setosus and in the fossil Chlamydotherium. 
The latter shows the fusion of two conical points. The author 
refers also to Glyptodon, which has prismatic back teeth with 
a very evident triconodont type. He concludes by saying 
that these circumstances afford a very marked support for the 
opinion, that the present biconodont teeth of these Edentata, 
at least the posterior members of the series, have arisen by 
reduction of a typical triconodont type. If this is confirmed 
by further observation, it will be a fact of the very greatest 
importance in the phylogeny of the mammalia. For the tri- 
conodont type has been shown to prevail in the upper Triassic 
and lower Jurassic periods. There was unquestionably a vast 
period in the evolution of mammalia in which a triconodont 
molar was the only type. 
SAUROPSIDA. In the conclusion of his work upon the 
development of the teeth in the crocodile, he reaches the fol- 
lowing result: “The cause of the existence of the thecodont 
(fangless) teeth is to be found in the continuous growth of the 
epithelial sheath of Hertwig. . . . -+ . . The fant- 
tional tooth of the crocodile is altogether analogous with a 
rooted mammalian tooth, the growth of which is not yet com- 
plete. The first embryonic tooth series of the crocodile, on, 
the other hand, exhibits a development which we hitherto 
have only observed in the Selachia and Urodela through the 
labors of Hertwig. The theory of Hertwig as to the genesis 
of the mouth skeleton out of the cement sockets of tooth 
structure, has received an unexpected and weighty support by 
our researches, ” 
The types examined by the author, included Sterna Wil- 
sonii, the Ostrich, Struthio camelus and the turtle, Chelone 
