OCR Text |
Show 1900.] STRUCTURE OF THE MUSK-OX. 713 column. It is also very easy to understand how the short basal tubercles in primitive forms, with comparatively short teeth, for this reason are transformed to columns in higher, more specialized, forms with long teeth. At the same time that the teeth are developed from brachydont to hypselodont, the basal or accessory elements are drawn out in length. The " Basalwulst" does not exist any longer in the hypselodont teeth. The formerly basal elements must therefore take their origin at the vertical surface of one of the main lobes, and become easily soldered to them, as is the case in the mandibular molars of tbe Boviuae. In this way the short basal tubercles are gradually changed into long columns, which together with the teeth themselves may become enlarged and add to the grinding-power of the teeth. Under such circumstances it should be only their situation at the inner side of the upper true molars that should guarantee to the accessory elements of these teeth the rank of " accessory columns," * and something different from the common " basal tubercles." This is, however, far from sure. Biitimeyer regards the, as he thinks, true accessory columns as " Abgliederungen vom Vorjoch dieser Zahne " (Anoplotherium molars). In the young, not yet fully developed, first molars of tbe common calf I have seen the beginning of the accessory columns connected with the posterior lobes. Zittel2 thinks that Anoplotherince have become extinct without leaving any progeny. Dichobunince, on the contrary, characterized by the same author as having " Obere M . fiinfhockerig, die Zwischenhocker in der hinteren Beihe," are regarded as ancestors of the now existing ruminants3. The latter opinion agrees consequently better with the condition found in the common calf. But it is not so with all ruminants : in Cephalophus, for instance, the accessory elements of some of the upper true molars belong very plainly to the anterior lobes, and tbe same is the case with all Ovibos molars. It is thus apparent that the place of origin of these accessory elements of the upper molars varies a good deal, and consequently cannot in any higher degree be taken into account for systematic purposes. It may also justly be questioned whether the " true " accessory columns are to be regarded as " Abgliederungen " from the primary lobes of the teeth ; and, if they should be so, what are then the common basal tubercles ? I do not hesitate to express the opinion that both the accessory columns and the basal tubercles have a similar origin, namely, are more or less developed basal tubercles4. 1 But in some instances not even this is sufficient, since Riitimeyer means that the accessory elements on the inner side of the molars in the Giraffe and the Deer ought to be regarded as " Basalwarzen " (I. c. p. 79). 2 Handbuch d. Palaontologie, I. iv. 3 This theory seems later to have been accepted by Riitimeyer too. 4 A similar opinion seems also to be accepted by later authors. Rose and Bartels in their paper " Ueber die Zahnentwicklung des Rindes" (Morph. Arb. herausg. v. Schwalbe, Bd. 6) shortly remark (I.e. p. 97) about "dieeigenartigen Basalpfeiler und Faltenbildungen...welche besonders die Zahne der Rinder so ausserordentlich kompliciren," that these are "ganz nachtragliche Ausstulp-un^ en des Zahnbeinkeimes." |