Dentition of Mammals. 281 



liaii class*, have, as is well known, <^enn.s of tectli in tli(i first 

 third of iheir ffietal life; these are subsequently absorbed. 

 Among recent investigators Julinf and Weber} widened the 

 difference which Eschricht§ previously stated to exist between 

 the nine anterior teeth and tlie posterior ones, by affirming 

 that the latter are not simply conical but have several cusps, 

 and that the dentition is absolutely heterodont. 



My own investigations were carried out upon thirty dif- 

 ferent specimens of large jaws of foetal Whalebone Whales, 

 including Megaptera boopSj Balmnoptera rofitrata, Baheno- 

 jytera Sibbaldii, and Balcenoptera niusculus, which were partly 

 preserved whole and ])artly divided into series of sections 

 made in the three chief directions. In the first place I dis- 

 ])Ute such a difference as has been stated to exist between the 

 nine anterior and the posterior teeth; the appearance of teeth 

 which seem to have several cusps is, in my preparations ot 

 older jaws, occasioned by the process of ab3or[)tion, which 

 begins at the tip||. Tiie posterior teeth are somewhat more 

 convex than the anterior ones, but throughout are simply 

 conical, with the exception of cases, which are of (piite isolated 

 occurrence, where a pair of neighbouring teeth are apparently 

 fused together. The position of double teeth of this kind 

 (three separate teeth or even four may also be united together) 

 scarcely follows any definite rule ; in a few cases they also 

 occur among the first nine teeth, and even on this account 

 they cannot correspond to the supposed molars, according to 

 Julin's interpretation. Are these double teeth secondary 

 fusions, or do they represent primitive conditions ? Embryo- 

 logy furnishes the answer. A series of seven embryos of 

 Balcenoptera miisculas^ measuring from 43 to 82 cm. in length, 

 shows that the number of tiie double teeth diminislies con- 

 siderably with increasing growth, while the number of the 

 separate tooth-tips in each half of the jaw remains constant 

 at fifty-three. In the youngest stages nine or even fifteen 

 teeth are fused together ; in the following ones five, four, and 

 three, and in the oldest only two. The same result, the 

 diminution of the double teeth with increasing growth, is 

 furnished by the comparison of younger and older embryos of 

 other species of Whalebone Whales. It follows from this 

 * W. Kulcenthal, " Ueber die Anpassung von Siiugetieren an das Lebeu 

 im Wasser," Zoologisclie Jaliibticber, 1890; Ann. and Mag. Nat. Hist. 

 ser. 6, vol. vii. pp. 153-179. 

 t Juliu, loc. cit. 

 \ "Weber, loc. cit. 



§ Eschricbt, ' Untersuebungen iiber die novdiscben "Waltiere ' : Leipzig, 

 1849. 



II Vide also Poucbet et Cbabrv, '' Sur revolution des dents des Balaen- 

 ides," Conipl. Rend. Ac. So. Paris, to.iie 94, uo. 8, pp. 540-542. 



Ann. d- Ma(/. y. Uidt. Ser. G. To/, ix. 21 



