OCR Text |
Show 700 DR. C. I. FORSYTH MAJOR ON THE [June 1, produce the well-known changes in the shape of the mandibula.- The angular process is less curved inwards than in the Malagasy genus.-Occiput inclined forwards. Basis cranil.-In Brachyuromys we have the conditions normal in Muridae; tbe tympanic bones are remarkable for their small size. Rhlzomyes: Auditory meatus tubular. The space behind the choanas, whose base is formed by the basisphenoid, is much compressed laterally. Braudt, in his description of the skull of the genus Rhizomysl, comprehending both Tachyoryctes and Rhlzomyes2, makes a statement apt to be misleading. Eeferring apparently to the figure of " Nyctocleptes dekan" (= Rhizomys sumatrensls) by Temminck,3 he says :-" Die innern Fliigelfortsatze des Keilbeins divergiren so stark nach aussen, dass hinten, wie bei keiner andern der bekannten Spalacoiden-Oattungen, eine sehr breite Gaumenrinne entsteht." All depends what extension i3 given to the term ' Gaumenrinne' (palatal groove). The internal pterygoid processes in fact diverge much with their inferior borders, so that, as is shown in Temminck's figure, and in Pl. XXXVIII. fig. 3 of the present paper, they almost completely cover the pterygoid fossae. Upwards, however, they converge very much; and lastly, in the upper third of their height, they run parallel with each other, so as to delimit a groove-whose bottom (or roof) is occupied by the basisphenoid-which is very deep anteriorly and whose upper portion is exceedingly narrow (Rh. sinensis). In Rh. sumatrensls the groove is slightly shallower and less narrow ; and the same, although in a minor degree, holds good with regard to Rh. prulnosus. In Tachyoryctes the groove in question is less deep; backwards, where it is no more bordered by the internal pterygoid processes, it appears as a very narrow longitudinal groove in the middle line of the basisphenoid, which is thus partitioned in two lateral portions; whereas in Rhizomys it has only somewhat raised borders. Anteriorly, the "palatal groove" appears somewhat deepened and enlarged, as shown in Eiippell's plate4 and in Pl. XXXVIII. fig. 3 of the present paper. In consequence of the vertical extent of the maxillary in Rhizomys, the bony palate comes to be situated at a very deep level. As a consequence, the external pterygoid processes also acquire a considerable vertical extension, since they serve as a "support for the maxillary." 5 But this is not the only explanation of the great depth of the pterygoid fossae ; otherwise their roof-or rather their bottom, as we are speaking of them as ' fossae'-would remain on a level with the inferior basis of the intervening basisphenoid. In Rhizomys and Tachyoryctes the anterior portion of the basisphenoid becomes increased in vertical size, forming a septum 1 J. F. Brandt, " Untersuchungen iib. d. craniolog. Entwicklungsstufen d. Nager d. Jetztzeit," Mem. Acad. St. P6tersbourg, [6] ix. vii. 1855, p. 817. - lb. p. 306. 3 ' Monographies de Mammalogie,' ii. Leiden, 1835, p. 40, pi. xxxiii. fig. 5. 4 Mus. Senckenberg. iii. Taf. x. fig. 2 c (Rhisomys [Tachyoryctes] macro-ccphalus). 8 Winge, Meddel. Naturh. Foreu. 1881, p. 40. |