H. F. shorn — Mammalia in North America. 389 



for the one advocated by Baume, Julin, Weber, and Winge, 

 that the multiple cetacean teeth represent the intercalation or 

 joint appearance of both the first and second series of teeth, 

 owing to the elongation of the jaw — a view which is now dis- 

 proved by Kiikenthal's discovery of the second row beneath 

 the first. Since even by Kiikenthal's hypothesis the typical 

 Mesozoic mammals could not furnish as many teeth as are 

 found in some of the dolphins, a likelier explanation than his 

 seems to be that as the jaws were elongated the dental fold 

 was carried back and the dental caps were multiplied. 



The Edentates, like the Cetaceans, point back to hetero- 

 dontism, and somewhat less clearly to a typical dental formula. 

 We are here indebted to Flower, Rheinhardt, Thomas, Kiiken- 

 thal, and Rose. It is their rudimental and useless first series 

 which gives the evidence of heterodontism, while the second 

 series has become adaptively rootless and homodont. The 

 especially aberrant feature is that a double succession exists in 

 the typical "true molar" region. The adult nine-banded 

 Armadillo presents only eight maxillary teeth, seven of which 

 are preceded by two-rooted milk teeth (Tomes) ; in the embryo 

 Leche finds fifteen dental caps, of which only thirteen are 

 calcified ; this number probably includes the four rudimentary 

 incisors observed by Rheinhardt. In the aberrant Oycteropus 

 (Aard-Yark), with ten adult teeth, Thomas finds seven milk 

 teeth behind the maxillary suture (thus taking us into the 

 molar region of the typical heterodonts). The last of these 

 milk teeth is large, and two-rooted ; behind this are three large 

 permanent posterior teeth, apparently belonging to the first 

 series. The large lateral tooth of Bradypus is suggestive of a 

 canine. From this rapidly accumulating evidence it appears 

 probable that the ancestral Edentates had four incisors, a canine 

 and eight or more teeth behind it, the double succession ex- 

 tending well back so that the first series did not become perma- 

 nent at the fifth tooth behind the canine as in the Marsupials 

 and higher Placentals. If these are primitive conditions, as 

 seems probable from comparison with fossil Edentates, they 

 carry the divergence of the Edentates, like that of the Ceta- 

 ceans, back into the Mesozoic period. Comparative anatomy 

 and embryology thus point back to highly varied branches of a 

 generalized placental heterodont stem in the Mesozoic, and a 

 much earlier divergence than we formerly imagined. Now 

 let us see what the early Mesozoic mammals point forward to. 



There are three distinct and contemporary Jurassic types, 

 the Multituberculates, the Triconodonts, and the Trituber- 

 culates. Are not these the representatives of the Prototheria, 

 Metatheria, and Eutheria? In the archaic Multituberculates 

 we have seen a monotreme type of jaw and vestiges of a 



