1880.] MR. W. A. FORBES ON THE UAKARI MONKEYS. 641 



(scissure oecipitale externe) first appears externally. It is at first 

 slightly concave forwards, then convex. At about" two thirds of its 

 course it is joined by the well-marked antero-temporal sulcus {a.t.) 

 {scissure parallile, Gratiolet; premier sillon temporal, Broca), which 

 commences near the lower margin of the temporal lobe. This is 

 a condition not found in Ateles, Lagothrix, Mycetes, or Pithecia 

 monachis ; it is represented by Gratiolet as existing in Cebus capu- 

 cinus (Mem. Plis Cer. Atlas, pi. x. figs. 7 & 8), and likewise occurs 

 in Ci'nocephahts, Macaciis, and other Old-world genera. 



Embracing the upper extremity of the Sylvian fissure is a some- 

 what Y-shaped sulcus, the "stem"' oftheYbeing short and springing 

 from the median line, whilst the two arms are much longer and run 

 downwards and outwards, and in the case of the anterior one forwards 

 as well, onto the external surface of the hemispheres, appearing 

 there one on each side of the upper part of the Sylvian fissure (fig. 9 ). 

 The posterior of these is, no doubt, the aforesaid temporo-occipital 

 sulcus {t.o.) (scissure perpendiculaire ear^erae of Gratiolet, scissure 

 oecipitale externe of Broca). The anterior limb no doubt corre- 

 sponds to the anterior part of the " sulcus (4) bounding the upper 

 border of the angular gyrus, having the form of a broad pointed 

 arch," described by Prof. Flower in PitJiecia tnonachts (I. c. p. 330). 

 A comparatively slight modification of the condition of these parts 

 in that species as represented in his fig. 1 would bring about that 

 which obtains in Brachi/urus, which also is represented in Gratiolet's 

 figures of Cebus capucinus and C. apella (I.e. pi. x. figs. 7 & 11). 

 This "supraangular" (s.a.) sulcus defines anteriorly the veell- 

 developed angular gyrus. The union of the temporo-occipital and 

 occipito-parietal (the two parts of the scissure oecipitale of Broca) 

 sulci divides off perfectly the occipital and parietal lobes ; so that 

 there is here no such superficial "pli de passage" betvveen these 

 two lobes as exists in Cebus capmcimis and Pithecia satanas. In 

 P. monachus there is, in addition, a second, more superior passage- 

 fold between these two lobes (Flower, /. c.f. In Cebus apella 

 (according to Gratiolet's figure. I.e. pi. 10. figs. 12, \2ljis), as iu 

 Brachj/urus, the " pli de passage " is concealed, so that superficially 

 the two lobes appear perfectly distinct. 



Anterior to the supraangular sulcus, the fissure of Rolando (p.p.) 

 (postero-parietal, Huxley) is seen as a well-developed sulcus forming 

 a sigmoid curve extending very nearly to the middle line. Ante- 

 rior to this is the superofrontal sulcus («./.), also curved, though 

 roughly parallel with the external border of the hemispheres. Tlie 

 orbital surface of the hemispheres is also marked by a somewhat 

 H-shaped complex sulcus (incisure en-U). The occipital lobe is 

 alm_ost entirely smooth ; below a sulcus is observable, curving up- 

 wards, and ending in a bifurcate manner in front of the lower termi- 

 nation of the occipito-temporal sulcus. Posterior to this is a very 



^ As regards this and sundry other diifei-ences between Pithecia monachus 

 and P. satanas, it must be remembered that the two are not perhaps strictly 

 congenei-ic, P. satanas liaving been separated, with P. chiropotes, as a genus 

 Chiropotcs. 



