388 H. F. shorn — Mammalia in North America. 



motely the degenerate modern Monotreme jaw. All we can 

 say, therefore, is that the Multitubereulates are an archaic 

 group, highly specialized even in the Trias, that they were 

 probably Monotremes, and neither structurally nor functionally 

 akin to the Diprotodont Marsupials (Owen) nor to the Micro- 

 biotheridee (Ameghino). With a dental mechanism and a con- 

 dyle exactly like that of the rodents, they show no trace of 

 canines, and the mode of evolution of their peculiar molars 

 was probably paralleled later in the rodents. They present 

 vestiges of a primitive dental formula, like this: 13 Cf P4:. 

 M± + . Thlaeodon shows C 1, P 4, MS. Thus, so far as this 

 doubtful paleontological evidence goes, the Monotremes had 

 a typical formula. 



Our next step is to unify the typical 5, 1, 3, 4 of recent 

 Marsupials with the 3, 1, 4, 3 of higher Placentals. .Thomas 

 has shown in his studies of recent Marsupials that they have 

 probably lost one of the four typical premolars (pm. 2) : this 

 observation, fortunately, is partly confirmed by Rose's finding 

 an embryonic germ of this tooth. Ignoring the incisors of 

 the Jurassic Marsupials, Thomas raised the number of ances- 

 tral incisors to five, the highest number known among recent 

 Marsupials ; Rose therefore made another step towards uni- 

 formity when he showed that the Marsupial i. 5 is probably a 

 member of the second series of incisors, and should not be 

 reckoned with the first. Now, if we suppose that the Pla- 

 centals have lost one incisor, and one molar (abundant evidence 

 of which is found in Otocyon, Centetes and Homo), we derive 

 as the ancestral formula of both orders : 



Incisors, 4 ; Canines and Premolars, 5 ; Molars, 4. 



The aberrant placental Cetacea point in the same direction 

 as we read in the conclusion of Weber's fine memoir : " All 

 the Cetacea sprang from a stem with a heterodont, but only 

 partly specialized dentition (something like that of Zeuglodon, 

 3, 1, p & m : 7), . . . not direct from Carnivores or Ungulates, 

 but from a generalized mammalian type of the Mesozoic period, 

 with some affinities with the Carnivora. . . . Zeuglodon itself 

 branched off extremely early from the primitive line, and the 

 heterodont Squalodon (mark its formula, 3, 1, 4, 7), " branched 

 off later from the toothed whale line, after the teeth had begun 

 to increase in number and before homodontism had set in." 

 It would be easier for us while speculating to take Squalodon 

 and the Odontocetes directly from the Jurassic mammalian 

 formula (3, 1, 4, 8). As for the multiplication of this formula, 

 we have found the way, says Kiikenthal, by which numerous 

 homodont teeth have arisen from a few heterodont molars, 

 it is by the splitting up of the numerous triconid molars of 

 Jurassic ancestors into three. He substitutes this hypothesis 



