238 



NATURE 



[January 4, iJ^94 



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

 specialised dentition (something like that of Zeuglodon, 

 73. Ci. P&'M). . . . not direct from Carnivores or Ungulatts, 

 but from a generalised mammalian type of the Me^ozoic 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. i.\i\. 7.) " branched 

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

 to increase in number and before homodontism had st-t in." It 

 would be easier for us while speculating to take Squalodon and 

 the OJontocetes directly from the Jurassic mammalian formula 

 (3. I. 4.8.). As for the multiplication of this formula, we have 

 found the way, says Kiikenthal, by which numerous homodont 

 '^tth have arisen from a few heterodont molars, namely, l>y the 

 splitting tip of each of the molai's of the Jurassic ancestors 

 into three. He substitutes this hypothesis 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 



i/(/(/(/ vuuuUUUij'-^ 



^-mmfmwmmr-- 



Ritahons nf the First and Second Series of Teeth, 



I. Reptiles. //. Marsupials. ///. Insectivores (Erinaceus> 



/K. Higher Placer.tals. /'. Edentates. VI. Cetacea, Odoiitocetes 



jaw — a view which is now disproved 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 ex- 

 planation than his S' ems to be that as the jaws were elongated 

 the dental fold was carried back and the dental caps multiplied. 

 •^ The Edentates, like the Cetaceans, point back to diphyo- 

 donti'im, 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 

 whjch 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 



NO. 1262, VOL. 49] 



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

 the embryro Leche finds fifteen dental caps, of which onlv 

 thirteen are calcified ; this number probably includes the four 

 rudimentary incisors f>bserved by Rheinhardt. In the aberrant 

 Orycteropus (Aard-Vark), 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 

 prohable that the ancestral Edentates had four incisors, a canine 

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

 ing well back, so that the first series did not become permanent 

 at the fifth tooth behind the canine, as in the Marsupials and 

 higher Placentals. If these are primitive conditions, as seems 

 probable fiom comparison with fossil Edentates, they carry the 

 divergence of the Edentates, like that of the Cetaceans, back 

 into the Mesozoic period. Comparative anatomy and embry- 

 ology thus point back to highly varied branches of a generalised 

 placental heterodont stem in the Mesozoic, and a much earlier 

 divergence than we formerly imagined. Now let us see to what 

 the early Mesozoic mammals point. 



There are three distinct and contemporary Jurassic types, the 

 Muliituberculates, the Triconodonts, and the Trituberculates. 

 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 typical ancestral 

 formula. The Triconodonts are a newer group, pei haps derived 

 from the Domotheriidte (incipient Triconodont^) of the Trias, 

 although these appear to be aberrant ; the typical forms extend 

 from Amphilestes to Triconodon, and exhibit the first stages of 

 development of the inflected Marsupial jaw. The Trituber- 

 culates include the Amphitheriidae and Amblotheriidse with 

 true tuberculo-sectorial lower molars, like those of modern 

 Insectivores; they alone exhibit the typical angular placental 

 jaw — no reason can be assigned for calling them Marsupials, 

 excepting the traditional reverence for the Marsupial stem 

 theory. Now, it is very significant that the avi rage dentition of 

 these old hut highly diverse forms, namely, Multituberculates, 

 3. ?4. 6., Tricono<ionts, 4. 1.4. 7., Trituberculates, 4. 1.4-5.8., 

 is also the dentition to which the existing mammals apparently 

 revert. 



The third problem is from what type of molar tooth did the 

 mammalian molar diverge? 



[To be continued.) 



SCIENTIFIC SERIALS. 



Sy moils' s Monthly Meteorological Magazine, December 1893. 

 — In an article entitled " March to October, 1893,'" Mr. Symons 

 deals with the temperature of the last eleven years in the north- 

 west of London, with instruments identical in themselves and 

 in exposure, with especial reference to the excrpiional summer 

 of the jear 1893. The first two tables give the average monthly 

 maximum in the shade, and the average maximum in the sun 

 fur 1883-92, compared with the mean values for 1893. In both 

 cases the means of 1893 exceeded the average in each month, 

 the excess of the shade maximum ranging from l'^'2in September 

 to 9°'7 in April, while the sun maximum shows an average 

 excess of 7°"i, ranging from i'8 in July to io°8 in May. The 

 tables showing the extreme maxima in shade and sun for each 

 month of 1893, with the average of the highest reading in the 

 ten corresponding months, again have plus signs ia every 

 instance in 1893, the greatest excess of the former being io°"5 

 in April, while the mean of the eight months (March to October) 

 shows an excess of 6°. The severe test of comparing the highest 

 reading for each month of 1893 with the absolute highest 

 reading in the corresponding month during the preceding ten 

 year^, shows that the season as a whole was unpiecedented. 

 The shade maxima weie unequalled in April, June, July and 

 August, the excess in April amounting 10 5 '4, whereas in no 

 other year of the ten have unequalled shade maxima occurred 

 in more than two months. In some particulars August 1893 is 

 unparalleled in thirty-six years The shade temperature at gh. 

 a.m. on the i8ih, viz. 84" 3, was 3°-5 higher than any other gh. 

 a.m. rea<iing, and the shade maximum on the l6ih to igih all 

 exceeded 90°, the only instance of this temperature being reached 

 on three consecutive days. 



