Dentition of Mammals. 281 
lian class *, have, as is well known, germs of teeth in the first 
third of their fetal life; these are subsequently absorbed, 
Among recent investigators Julint and Weberf widened the 
difference which Eschricht§ previously stated to exist between 
the nine anterior teeth and the 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 hoops, Balenoptera rostrata, Baleno- 
ptera Sibbaldit, and Balenoptera musculus, which were partly 
preserved whole and partly divided into series of sections 
made in the three chief directions. In the first place I dis- 
pute 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 absorption, which 
begins at the tip||.. The posterior teeth are somewhat more 
convex than the anterior ones, but throughout are simply 
conical, with tie exception of cases, which are of qu'te 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 ? Kmbryo- 
logy furnishes the answer. A series of seven embryos of 
Balenoptera musculus, measuring from 43 to 82 cm. in length, 
shows that the number of the double teeth diminishes 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, 1s 
furnished by the comparison of younger and older embryos of 
other species of Whalebone Whales. It follows from this 
* W. Kukenthal, “Ueber die Anpassung von Saugetieren an das Leben 
im Wasser,” Zoolo wische Jahrbiicher, 1890; Ann. “and Mag. Nat. Hist. 
ser. 6, vol. vil. pp. 153-179. 
+ Julin, loe. cit. 
t Weber, loc. ett. 
§ Eschricht, ‘ Untersuchungen tiber die nordischen Waltiere’: Leipzig, 
1849. 
\| Vide also Pouchet et Chabry, “ Sur l’évolution des dents des Baleen- 
ides,” Compt. Rend. Ac. Sc. Paris, tone 94, no, 8, pp. 540-542, 
Ann. & Mag. N. Hist. Ser. 6. | Tale xe Bil 
