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Show 1899.] THE CARPUS OF CTENOMYS. 435 position corresponds perfectly with one which in other Marsupials (e.g. Petaurus and Trichosurus) is perfectly distinct and is interpreted as lunatum (intermedium)."' In the stages figured on plate 33 (tigs. 5 & 6) the element of Phascolarctus is not entirely independent, " sondern bereits dem Radius angewachsen.-In weiter ausge-bildeten Stadien finde ich keine Spur von eiuem solchen Element mehr, aber der Radius besitzt an der entsprecbenden Stelle einen mehr oder weniger deutlichen Vorsprung, den ich als dessen Homologon betrachten mochte." Now, not only the position of this element of Phascolarctus, but also what Emery states about its subsequent fusion with the radius, correspond so exactly with what I find in the above-named Rodents, that both appear to be homologous. The so-called scapho-luuar of Ctenomys, Mus, Brachyuromys, &c. would then at first sight seem to be a greatly enlarged scaphoid, which has overtaken the functions of the lunar, the latter having become reduced and eventually fused with the radius. Whenever we find in the carpus or tarsus of a species or whole group a large bone occupying the same place as two smaller bones in another, the conclusion nearest at hand is that the single bone is the result of the fusion of two originally distinct ossicles. But this inference is by no means always valid. I have elsewhere undertaken to demonstrate that the hamatum of Mammalia is not a compound of carpale 4 and carpale 5, but is carpale 4 only; for the obvious reason that there is a carpale 5, which however is generally cut away in the skeletons, being considered as a despicable sesamoid. In other instances it either vanishes or becomes fused with the tuberosity of the fifth metacarpal; it fuses with carpale 4 only in the case of a few Cetacea. I will here give another remarkable instance of a similar kind. In tbe small Rodent group Bathyergiuse, the genera Bathyergus and Georgchus (capensis) exhibit in their carpus a distinct ossicle, which from its position we call centrale; proximad it articulates chiefly with the equally distinct lunatum, and distad with the third and second carpale (magnum and trapezoideum). In the closely allied Myoscalops there is, occupying the place of the centrale and the trapezoideum of the former two genera, only one bone, which runs obliquely from the lunatum to the carpale 1 (trapezium) and, on its way, articulates also with carpale 3, as does the centrale of the two fore-named genera, and with the scaphoid and ineta-carpale II., as does the trapezoideum of Bathyergus and Georychus. In the tarsus of the same genera occurs the following curious parallel. In Bathyergus and Georychus the navicular is separated from the second metatarsal by the tarsale 2 (mesocuneiforme); in Myoscalops the navicular encroaches on the space occupied by the mesocuneiforme of the former two genera and articulates with the second metatarsale; so that the mesocuneiforme seems to be missing in Myoscalops. The obvious inference from this condition will of course be that the single bone in the carpus of Myoscalops is a 1 L. c. p. 373. |
