1. _Ileo-colic junction marked externally by an annular constriction, corresponding to a ring-valve with central circular opening in the interior_ (Fig. 465, _I_, 1). This form is encountered in many Teleosts. The projecting annular mucous fold resembles the pyloro-duodenal valve. Figs. 311-315 illustrate the structures in representative Teleosts. Among the higher forms this type of ileo-colic junction is encountered in the simple alimentary canal of many Amphibians (Figs. 318-320). Among Reptiles it is found in certain lizards, as in _Heloderma suspectum_, the gila monster (Fig. 322). This animal lives almost entirely upon bird's eggs, and its simple and reduced ileo-colic junction contrasts strongly with the highly developed and complicated caecal apparatus of the phytophagous lizards, as _Iguana_ (Figs. 326-330), affording one of the most striking illustrations of the effect which the character of the food habitually taken has on the structure of the alimentary canal in forms otherwise closely allied. The same type of ileo-colic junction, as a reduction form, occurs in the arctoid group of Carnivora among Mammalia (cf. p. 212). 2. _Differentiation in caliber of large and small intestine. Funnel-shaped ileo-colic transition._ This type, compared with the preceding, is characterized (Fig. 465, _I_, 2) by the greatly increased caliber of the large intestine, resulting in a funnel-shaped transition between mid- and hindgut, the small intestine continuing into the colon at the apex of the funnel. Examples of this type are presented by several Edentates, _Myrmecophaga jubata_, the great ant-eater (Fig. 356), and _Choloepus didactylus_, the two-toed sloth (Fig. 357). 3. _Abrupt demarcation of small and large intestine with caliber differentiation_ (Fig. 465, _I_, 3). The small intestine is still central at the ileo-colic junction, _i. e._, the axis of its lumen is continuous with the central axis of the colic lumen. In place of the gradual funnel-shaped transition of the preceding type the demarcation is abrupt. An example of this form is furnished by another Edentate, _Tatusia peba_, the nine-banded armadillo (Fig. 358). Among reptiles a similar well-marked ileo-colic transition is encountered in _Alligator mississippiensis_ (Fig. 321). 4. _Colic pouch prolonged back on each side of the ileo-colic junction, producing symmetrical colic caeca_ (Fig. 465, _I_, 4). A growth of the colic tube cephalad, on each side of the junction with the midgut, leads to the formation of this type, characterized by the presence of two symmetrical globular caecal pouches. In its simplest form this condition is illustrated by the double colic caeca of another armadillo, _Dasypus sexcinctus_ (Fig. 359). The bifid caecal apparatus of the American manatee (Fig. 366) belongs to the same group.