adjacent margin of the ileum, nearer to the dorsal larger than to the ventral smaller caecal artery. We have, therefore, in the three types just considered, the following variations in the arrangement of the vascular and non-vascular folds: 1. (_a_) Ventral and dorsal vascular folds distinct } and free. Ventral and dorsal caecal } arteries of nearly equal size. } (_b_) Intermediate non-vascular fold free on }_Ateles._ both surfaces, placed nearer to the } dorsal than to the ventral vascular fold. } 2. (_a_) Ventral vascular fold distinct. Ventral } caecal artery somewhat further reduced } in size. Dorsal vascular fold distinct } only over the dorsal surface of the ileo-colic} junction. At the lower border of } the ileo-colic junction the dorsal vascular }_Mycetes._ fold fuses with the intermediate non-vascular } fold. } (_b_) Intermediate non-vascular fold free only } on ventral surface, the dorsal surface } below the ileo-colic junction being fused } with the dorsal vascular fold. } 3. (_a_) Dorsal and ventral vascular folds reduced. } Dorsal artery much larger than ventral. }_Cercopithecus._ (_b_) Intermediate non-vascular fold well developed,} free on both surfaces. } We may judge from this series that the following factors are capable of materially modifying the definite arrangement of the structures: 1. The vascular folds are capable of reduction until the vessels run close to the intestinal surface, merely covered by somewhat redundant peritoneum containing epiploic appendages. (_Cercopithecus._) 2. The dorsal caecal artery tends to assume in all three forms the greater share in the caecal vascular supply. This tendency is slightly developed in _Ateles_, becomes more pronounced in _Mycetes_, and is well marked in _Cercopithecus_, in which animal the dorsal vessel nearly replaces the ventral branch, the latter confining itself to the ventral surface of the ileo-colic junction and the adjacent ventral parts of the caecal wall. 3. The intermediate non-vascular fold is placed nearer to the dorsal larger than to the ventral smaller caecal artery. This condition, present in both _Ateles_ and _Cercopithecus_, foreshadows the fusion of the intermediate and dorsal vascular folds at the lower border of the ileo-colic junction, as seen in _Mycetes_.