2S2 iJr. "\V. Kiikciithal on fie 



tliat the (li>ul)lc tcftli voprescnt an ovi^-inal coiidlrion, and are 

 tlicict'oro to be veoorded as molars, and further that CONICAL 



TEETH, WITH SINGLE TITS, ARISE FliOM AIOLARS BY DIVISION. 



We have thus learnt a method by which numerous homodont 

 teeth arise from a small number of hcterodont molars. E 

 shall subsequently adduce the pala!ontolog"ical facts which 

 substantiate such an oiigin of homodont from heterodont 

 dentitions; 1 would here only further allude in all Ijrevity to 

 an analogous ])henonienon which occurs in a Bearded Seal 

 [PJioca harhata) from Spitzbergen. 



Owing to mechanical causes (hard food, consisting of 

 mussels, besides the final reason, which is the incomplete 

 calcification of the teeth) tli(>. molars in the specimen before 

 me have worn away, ami, with the exception of the last, have 

 ( ach become more or less completely separated into two, which 

 present an absolutely similar appearance; instead of five molars, 

 we consequently find seven and eight unicuspid teeth. 



The results of my embryological investigations decide the 

 question whether the teeth of Whalebone Whales belong to the 

 first or the second series, in so far as they show that rudi- 

 ments of a second series of teeth are still present ; the cord of 

 e])ithelium in question is for the most ])art fused with the 

 enamel-germ of the actual tooth, which therefore essentially 

 corresponds to the first series. The teeth belonging thereto 

 resenible in this the so-called true molars of all other mammals, 

 which, as they have no precursors in the milk-dentition, are 

 assigned to the second scries, although they niust be regarded 

 as liaving arisen from the fusion of the rudiments of both 

 dentitions. (In the case of the first molar this is often still 

 distinctly demonstrable ; it is to be seen with especial 

 clearness in embryos of Spermophilus leptodactylus^ for 

 instance.) 



I refer the peculiar transformations of the dentition in 

 ])elagic mammals, which hiwa just been described, to mecha- 

 riical causes^ terminating with diminished calcitication, which, 

 as being necessary for the diminution of the specific gravity, 

 is a phenomenon of very frequent occurrence in pelagic 

 mammals, and, as has ah-eady been shown, also gave the 

 lirst stimulus which led to the occurrence of hyperphalangy, 

 as well as the loss of the dermal armature of the Toothed 

 Whales*. 



* In my pajiev on the '' Adaptation of Mammals to Aquatic fjife " (Zool. 

 Jahibiiclior, 1800 [Ann. ami Mag. loc. cif.]), I explaiiioil these views in 

 greater detail. Of tlie lormer jaesfnoe of a dermal arnniture in Toothed 

 Whales, which I inferred from grounds of comjiarative auatomy aiul 

 embryology (Anat. Anzeiger, 18U0. p. I'-S?). I am now able to adduce 

 palajontological proofs also. 



