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Show 1901.] LEMUR MONGOZ AND L. RUBRIVENTFR. 261 large measure towards the formation of the wall of the sinus sphe-noidalis ; or else this function may be taken over by an intercalar bone making its appearance in the most posterior part of the medial orbital wall and articulating with the same bones as does the " third element" noticed by Cleland. Gegenbaur states ' that the superficies sphenoidalis of the orbital process, situated behind the latter's superficies ethmoidalis, articulates with the body of the sphenoid, from the cavity of which a hollow (" Buchtung ") extends on to the sphenoidal surface, and he accordingly figures 2 a small cavity in this part of the orbital process, almost symmetrical with, but smaller than, the cavity of the ethmoidal surface. There is therefore uo perfect accord amongst the writers on this topic-Gegenbaur representing as the normal condition what by other anatomists is termed an occasional occurrence; and, like llenle before him, he does not speak of an articulation of the orbital process with the ossicula Bertini, but with the praesphenoid. In Lemurs-Lemur fulvus (" L. mongoz," B.M. Z. D. No. 60 b), Lepidolemur (see text-figure 67, p. 259), Avahis (see s., text-figure 27, above, p. 132)-I find sometimes present a separate bone, occupying exactly the positiou of Cleland's " third element" 3 and Toldt's " Schaltknochen." In one instance at least amongst Lemurs (see below under Lemur mbriventer), the anterior portion of the sphenoidal sinus is annexed in a later stage by a palatal sinus. I am likewise inclined to assume that the small cavity in the human orbital palatal communicating with one of the ethmoidal cells is the remnant of the condition in Lemurs, where ethmoid and palatal concur in forming a cavity. From this it follows that the so-called " cellulae ethmoidales " are not organites proper only to the ethmoid of Man, and hence are not without phylogenetical importance, as assumed by Seydel4. Proceeding now to an examination of the conditions in Lemur mongoz, I find that in all the skulls of this species which I had the opportunity to examine the pars perpendicularis and the orbital process of the palatal diverge considerably from behind forward, so as to enclose between them the posterior triangular portion of a spacious cavity. The divergence of the pars perpendicularis takes place by its advancing into the cavum nasale from behind forwards in a latero-medial direction, so that the two parts of either side converge slightly towards the middle line (PI. XXII. fig. 10, p.p.) ; each of them terminates freely with a sharp vertical margin, which forms the posterior boundary of the large opening of the cavity into the cavum nasale. The divergence of the orbital process is produced by its being inflated in a lateral direction. The vaulted 1 C. Gegenbaur, Lehrb. d. Anat, d. Menschen, 6te Aufl., i. p. 234 (1895). 2 Op. cit. p. 233, fig. 176 0. 3 " Articulating behind with the sphenoid, in front with the ethmoid, inferiorly with the palatals, aud sometimes [in Lemurs always-F. M.] above with the frontal." 4 Morpb. Jahrb. xvii. pp. 86, 89 (1891). P R O C ZOOL. Soc-1901, VOL. I. No. XVIII. 18 |