This unique arrangement, which is not found in any other known vertebrate, may possibly be led back to a type-form encountered in certain saurians (see p. 234). 4. SUBORDER: PROBOSCIDEA. The caecum of the elephant is a very large sacculated pouch with rounded termination, illustrated in Fig. 379, taken from the Asiatic elephant. [Illustration: FIG. 379.--_Elephas indicus_, Asiatic elephant. Ileo-colic junction and caecum. (Columbia University Museum, No. 995.)] VII. Order: Rodentia. With the exception of a single group, the dormice (_Myoxus_) (Fig. 380), the rodents possess a well-developed caecal apparatus. [Illustration: FIG. 380.--_Myoxus avellanarius_, common dormouse. Alimentary canal. (Columbia University Museum, No. 1466.)] In some forms the terminal portion of the pouch is reduced so as to constitute an appendix. Many of these animals, in addition to the caecum proper, have the proximal colon elongated and coiled in a spiral, and in some this part of the large intestine is provided in the interior with a spiral mucous fold. This latter structure functions again to increase the extent of the mucous absorbing surface and to prolong the retention of substances undergoing slow digestion and absorption. Typical examples of the capacious sacculated rodent caecum, with a terminal pointed reduced segment, are afforded by _Castor fiber_, the beaver (Figs. 381 and 382) and by _Erethizon dorsatus_, the Canadian porcupine (Figs. 383 and 384). Figs. 385 and 386 show the ileo-colic junction, caecum and appendix in _Lepus cuniculus_, the rabbit. The interior of the caecal pouch and of the proximal segment of the colon is provided with a complete spiral valve (Fig. 387), while the appendix is differentiated by the histological character of its mucous membrane which is studded with closely packed adenoid follicles (Fig. 388). A similar aggregation of lymphoid tissue is found in this animal at the ileo-colic junction forming the s. c. _saccus lymphaticus_ (Fig. 387). [Illustration: FIG. 381.--_Castor fiber_, beaver. Ileo-colic junction, caecum, and proximal colon; ventral view. (Columbia University Museum, No. 1607.)] [Illustration: FIG. 382.--_Castor fiber_, beaver. Ileo-colic junction, caecum, and proximal colon; dorsal view. (Columbia University Museum, No. 1607.)] [Illustration: FIG. 383.--_Erethizon dorsatus_, Canadian porcupine. Ileo-colic junction, caecum, and colon; ventral view. (Columbia University, Study Collection.)] [Illustration: FIG. 384.--_Erethizon dorsatus_, Canadian porcupine. Ileo-colic junction, caecum, and colon; dorsal view. (Columbia University, Study Collection.)] [Illustration: FIG. 385.--_Lepus cuniculus_, rabbit. Ileo-colic junction and caecum. (Columbia University Museum, No. 1568.)] [Illustration: FIG. 386.--_Lepus cuniculus_, rabbit. Ileo-colic junction with saccus lymphaticus, isolated, (Columbia University, Study Collection.)]