Type: Pithecia satanas (Fig. 452).
Fig. 452.—Pithecia satanas, black saki monkey. Ileo-colic junction and cæcum. (Columbia University Museum, No. 641.)
In general the Arctopithecini and Ateles, Mycetes, Lagothrix and Pithecia among the Cebidæ form a group containing a series of cæcal transition types which lead up to the anthropomorphous type, illustrating the following conditions:
(a) The inherent crescentic curve of the cæcum, with the concavity directed toward the left, and carrying the apex of the pouch upward toward the lower border of the ileum and the ileo-colic junction. (Hapalidæ, Ateles, Lagothrix.)
(b) The reduction in caliber of the terminal part, foreshadowing by the pointed and narrow extremity of the pouch the appearance of the appendix in the anthropomorphous group. (Hapalidæ, Ateles.)
(c) The constriction at the level of the ileo-cæcal junction, with the corresponding well-marked differentiation between cæcum and colon in the interior. (Ateles.)
(d) The sharp bend in the pouch as it makes its turn upward and to the left, repeated in certain types of adult human cæca (cf. [p. 247]). (Lagothrix.)
(e) Pithecia forms a transitive type between the blunt sacculated cæca of the Cynomorpha and the curved pointed pouches of the Cebidæ, partaking of the characters of both.
(f) The same character is seen in the cæcum of Mycetes fuscus the brown howler monkey ([Figs. 449] and [450]).