4. Direct continuity of small and large intestine, with lateral colic caecum, extending along the convex free border of the terminal ileum and slightly convoluted at the extremity (Fig. 465, _II_, 4), characterizes the entire group of the _Cebidae_ among the new-world monkeys. The caecum in these animals is a comparatively long pouch, nearly equalling in caliber the remainder of the intestine, occupying a distinctly _lateral_ position, with the terminal portion rounded and slightly recurved (Figs. 453 and 454). 5. The _Cynoid group_ of Carnivora, including the dogs, wolves, jackals and foxes, presents a similar relative position of small and large intestine and caecum (Fig. 465, _II_, 5). The caecum, compared with that of _Cebus_, is longer and more highly convoluted (Fig. 397). Variations encountered in certain forms indicate reversions to a more primitive type. Thus Fig. 398 shows the usual form in the dog, while Fig. 399 exhibits an occasional type in the same animal. The caecum here is less twisted and indicates the probable derivation of the more commonly encountered type. III. Rectangular Ileo-colic Junction with Direct Linear Continuity of Caecum and Colon. (Fig. 465, III.) The third general group, to which the large majority of Mammalia belong, is characterized in its typical form by a right-angled entrance of ileum into large intestine and by the direct caudal prolongation of the colon into a caecal pouch of nearly uniform caliber with globular termination. The axes of the small and large intestine are not in the same line as in the two former groups, but are placed nearly at right angles to each other. With this change in the direction of the main intestinal segments the caecum ceases to be a lateral appendage to the canal and appears as a caudal prolongation of the colon beyond the ileo-colic junction (Fig. 465, _III_). The type-form of this group is encountered among the herbivorous Marsupialia, such as the kangaroos and opossums. Fig. 350 shows the ileo-colic junction and caecum in the rock wallaby, _Halmaturus derbyanus_, and Fig. 348 the same structures in our common opossum, _Didelphis virginiana_. The majority of the remaining mammalian forms depend upon modifications of this type, either in the direction of reduction of the caecal apparatus, or of increased development with concomitant structural changes of similar physiological import in the proximal portion of the colon. The following subdivisions of the general group may be established. A. 1. The caecum is long, markedly curved or uncinate, with the crescentic medial margin turned toward the free border of the terminal ileum. The entire pouch usually diminishes gradually in caliber to its termination (Fig. 465, _III_, _A_, 1). This type is encountered in a large group of new-world monkeys, including the marmosets and howlers.