74 
Oldfield Thomas set forth an excellent description of the relation 
between the tooth-formulæ of the Placentalia and Aplacentalia. 
år 4o the tooth-forms he only says that the primitive mammalian 
teeth were probably similar to those of Dasypodidæ and perhaps 
homo- and monophyodont, with no fangs. 
The Concrescence Theory was in the year 1888 supported from 
Vienna by Dybowski. His starting-point was the ungulate molars. 
and he tried to show that the upper aud lower molars were sym- 
metrically constructed, but were so placed that the lateral (outer) 
+Jochwand” above corresponds with the medial (inner) wall below 
and -vice verså. ,,Dieses gilt fir alle Såugetierzåhne uberhaupt." 
With few exceptions every mammalian tooth is composed of four 
må'n parts ("Vor”- "Mittel”- "Nach”-. and "Hinterjoch”). It is 
icue, one or more may degenerate, but all teeth — including the 
incisors and canine — can be traced back to the "7ierjochige” 
construction. Every "Joch” consists of "Wand” and "Bogen". By 
their more or less intimate coalescence the different tooth-forms 
arise. This theory allows a strictly methodical homologising of 
every cusp and every fold. Each of the two parts of a "Joch” is 
composed of three more or less independent "Pfeiler”, every normal 
tooth thus contains 24 "Pfeiler”. There is every reason for be- 
lieving that every "Pfeiler'”” corresponds with one primitive Selachian 
tooth: ,,Einem jeden 4-jochigen Såugetierzahne liegen 24 einfachen 
Papillen zu Grunde, aus welchen ebensoviele Pfeiler entstehen, z. B. 
die Zåhne mit nicht zentralisiertem Zahnbeine. Indem nun je 3 
Pfeiler mit einander verwachsen, entsteht je ein Halbjoch, die 
ihrerseits untereinander verwachsend je ein Zahnjoch bilden, Aus 
dem Verwachsen einzelner Zahnjoche miteinander kommt eben der 
zusammengesetzte, vierjochige Zahn zustande. Durch das Verkiåimmern 
einzelner Pfeiler (resp. Papillen) erklårt sich das Verkimmern oder 
gar Fehlschlagen der einzelnen Zahnjoche," (pag. 7). 
In the same year Osborn's famous theory appeared.) This 
1) Of Osborn's many papers on this subject only those are cited which 
ve something essentially new; besides see the book 
of Osborn and Gregory. 
