Passing now from the condition presented by _Ateles_, with three fully developed and distinct ileo-caecal folds, to the next stage leading up to the normal human arrangement, we find the same illustrated in the caecum of another new-world monkey, _Mycetes fuscus_, the brown howler monkey, shown in the ventral and dorsal views in Figs. 449 and 450. The ventral vascular fold (Fig. 449, 1) is still well developed, the contained ventral branch of the ileo-colic artery descending over the ventral wall of the ileo-colic junction and caecum and supplying both. The dorsal vascular fold (Fig. 450, 2), on the other hand, is nearly completely fused with the intermediate non-vascular reduplication (Figs. 449 and 450, 3), the approximation between these structures exhibited by _Ateles_ having in _Mycetes_ reached the point of actual union, so that the larger dorsal branch of the ileo-caecal artery descends to the apex of the caecum in the following manner: The main post-caecal artery passes over the dorsal surface of the ileo-colic junction included in a short serous fold which corresponds to the dorsal vascular fold of _Ateles_. Beyond the lower border of the ileo-colic junction this fold fuses with the intermediate non-vascular fold, one arterial branch descending along the line of attachment of this fold to the caecum, the other distributed over the dorsal surface of the pouch. A third type, also taken from the lower Primates, is presented by the caecum of a cynomorphous monkey, _Cercopithecus sabaeus_, the African green monkey, shown in Fig. 432, in the ventral and left aspect with the terminal ileum lifted up. The caecum of this animal is comparatively short, somewhat conical, terminating in a blunt apex. The vascular supply is arranged on the same type as in _Ateles_ and _Mycetes_, _i. e._, a trunk of the ileo-colic artery divides at the ileo-colic notch, one branch descending ventrad, the other dorsad of the ileo-colic junction. The slightly larger size of the dorsal vessel, noted in _Ateles_ and _Mycetes_, has been increased in _Cercopithecus_ until the ventral artery (1) supplies merely the front of the ileo-colic junction and the upper part of the adjoining ventral wall of the caecum, while the larger dorsal vessel (2) descends behind the ileo-colic junction, supplying the same and the entire dorsal and apical portions of the caecum. The relation of these caecal arteries to the peritoneum is moreover different from that encountered in _Ateles_. In place of running in distinct mesenteric folds, as in the latter species, the vessels pass close to the surface of the intestine, merely covered and partly surrounded by slightly redundant visceral peritoneum containing numerous pads of epiploic fat, which bead the course of the vessels at regular intervals. Between the two arteries the intermediate non-vascular fold (2) is seen, presenting much the same arrangement as in _Ateles_ and passing between the left border of the caecum and the