79 
teeth are originally integumentary structures (Selachia), the teeth of 
the jaws somewhat specialised, but similar to each other; the num- 
ber of the tooth-rows is unlimited. In Batrachia and Reptilia only 
the teeth of the mouth-cavity remain, the number of dentitions is 
limited, the new rows are produced only as compensation -for the 
worn-out. In the higher reptiles the tooth-number may be constant 
and the single teeth somewhat specialised. The reptilian tooth-rows 
have in the primitive mammals become two dentitions, the milk- 
teeth and the replacing teeth, and the single teeth are furthermore 
differentiated:  ,,Bei der eintretenden Verkirzung der Kiefer rick- 
ten die Zahnkeime der einspitzigen Reptilienzåhne nåher und nåher 
aneinander und verschmolzen gruppenweise zu mehrspitzigen Zåhne, 
den urspriinglichen Backenzåhnen der' ersten Såugetiere. Durch 
die infolge verschiedener physiologischen Leistungen  geforderten 
Umformungen bildeten sich die Backenzåhne aus, wie wir sie bei den 
jetzt lebenden Såugetieren kennen." (pag. 483). Through the 
pålæontology the homologies of the single. tooth-cusps can be traced 
as easily as those of the fingers and toes on the limbs. — In 
"Entstehung und: Entwickelung des Såugetierstammes” 1891 Kiiken- 
thal calls attention to the teeth of the Multituberculata as evidence 
of an original fusion of several teeth into one; the multituberculate 
teeth are formed by one longitudinal row of functional reptilian teeth 
united with 1—2 rows of replacing teeth. Consequently they must be 
Present in a small number (not exceeding six). The cause of the 
tuncrescence is difficult to trace; it is not alone due to a shortening of 
the jaws. — From the multituberculate teeth both the teeth of 
the marsupials and of the placental mammals must be derived. — 
If this hypothesis be correct, the Cope-Osborn theory of cusp-addi- 
tion by the supplying of nourishment to certain parts of the tooth 
is nullified. »Nimmt man dagegen mit mir den triconodonten 
und tritubereularen Zahn nur als eine besondere Abteilung der 
Multituberculaten Zåhne, also als urspringlich durch Verwachsung 
entstandene Bildungen an, so ist die Schwierigkeit erhoben. Die 
Weiteren an den tritubercularen Zahntypus anschliessenden' Hypo- 
