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Show 1897.] MALAGASY GENUS BRACHYUROMYS. 701 between the two pterygoid fossae, which is reduced to a thin transparent plate. In this way is brought about the great depth and spaciousness of the pterygoid fosste. Their roof is covered by an irregular network of raised ridges, apparently for the pterygoid internus, greatly developed as a masticatory muscle. A beginning of these conditions we meet within the Microtince: in Fiber there is in the bony septum a spacious fenestra, situated farther backwards than the small fontanelle which is seen in Tachyoryctes. The Microtince are somewhat intermediate in this respect between the Rhlzomyes and typical brachyodont Muridae, including Brachyuromys. In the latter, the height of the basisphenoid is not increased ; it slopes considerably downwards from before backwards, so that the shallow pterygoid fossae are situated slightly below the level of the inferior surface of the basisphenoid, which broadly separates them. In Microtince they are situated somewhat above the level of the basisphenoid. Winge, speaking of the Bathyerglnl, remarks that the strongly developed m. pterygoideus which is inserted alongside the centra of the sphenoidea has transformed the presphenoid into a thin vertical platel. I find that both the presphenoid and basisphenoid are thus transformed, at least in the genera Bathyergus and Georgchus. 2. Spalax.-Winge places Spalax amongst the Dipodidae J on account of the form of the infraorbital canal, and for having m.l of the same size as m.2. H e states though, that besides the complete absence of premolars, this genus is in other respects as well on a somewhat higher level than the rest of the Dipodidae. The outer wall of the infraorbital canal is certainly not greatly developed ; and in adult specimens it is besides pushed considerably forwards, whilst at the same time starting almost horizontally from the cranium. The upper maxillary radix of the zygoma, which in Muridae usually forms the roof of the infraorbital canal, is more obliquely extended downwards in Spalax (not much differing from what obtains in Zapus), so that it helps to form the outer wall of the canal. The direction and extension of the walls, of course, shape the form of the canal itself. But with all that, the agreement with the Dipodidae is not so considerable, and besides seems to be a secondarily acquired character in Spalax; and this for the following reasons:- (1) In younger specimens of Spalax (B.M.), and in some adult, the outer wall of the canal is by no means pushed considerably forwards and neither is it horizontal, but more upright; as a consequence these specimens approach Siphneus in the form of the canal. 1 ' Gtnavere fra Lagoa Santa,' p. 127, and footnote 62, p. 169 : "Den Del af M. pterygoideus internus, der voxer op paa Siderne af Kllebenskroppene, er meget stark, naar langt frem under Foramen opticum og har omformet det forreste Kilebenskrop til en tynd lodret Plade." Footnote (p. 169): "De paa-gjaeklende Muskier ere gjennemgaaede bos Georychus capensis; Mterkerne paa Hovedskallen ere de samme hos de andre Slsegter." - ' Gnavere fra Lagoa Santa,' pp. 109, 121, 166. PROC. ZOOL. SOC-1897, No. XLVI. 46 |