4. This fusion of the two folds named in _Mycetes_ results in giving different values to the dorsal vascular fold in its proximal and distal segments. The proximal segment descends from the ileo-colic notch behind the ileo-colic junction to its lower border as a distinct fold. Beyond this point its fusion with the distal (caecal) segment of the intermediate fold rounds out a fossa, the inferior or posterior ileo-caecal, which is consequently bounded in front by the intermediate vascular fold, behind by the proximal segment of the dorsal vascular fold, to the right side by the inner wall of the caecum, between the intermediate and dorsal vascular folds, above by the lower border of ileum and ileo-colic junction, and below by the fusion of the two folds. This pocket or fossa which is the most important and constant of the peritoneal recesses in the neighborhood of the caecum, opens upward and to the left. 5. A superior or anterior ileo-caecal fossa, formed in cases of well-developed ventral vascular fold between the same and the ventral wall of the ileo-colic junction, is of small size and shallow. The cause of the greater development of the dorsal as compared with the ventral caecal artery is probably to be sought in the adhesion of the colon to the dorsal parietal peritoneum. In _Cercopithecus_ the dorsal surface of the ascending colon is adherent to the parietal peritoneum down as far as the iliac region and beginning of the caecum, whereas in _Mycetes_ the entire caecum, as well as the ascending colon, are free and non-adherent to the abdominal parietes. The influence of this adhesion on the arrangement of the vascular supply of the lower portion of the ascending colon and caecum appears to be important. Some of the departures from the _Ateles_ type presented by _Cercopithecus_ become still better developed in the human subject, where the adhesion of the ascending colon and the obliteration of the apposed serous surfaces of ascending mesocolon and parietal peritoneum is normally complete, even if the caecum remains entirely free, or only adheres to the iliac parietal peritoneum in the proximal part of its dorsal surface. Comparison with forms presenting non-adherent colic and caecal tubes indicates that the adhesion determines the relative size and arrangement of the ileo-colic vessels. Thus the partially adherent colon and caecum of _Cercopithecus_ presents, compared with the free tube of _Ateles_ and _Mycetes_, a marked reduction of the ventral and a corresponding enlargement of the dorsal caecal artery. Further progress in the same direction is noted in the human subject where normally the ascending colon and at times the proximal portion of the caecum are adherent to the dorsal parietal peritoneum.