384 ANATOMY OF VERTEBRATES. 



mandible when the mouth is closed. The side-walls of the mouth 

 are not dilatable and contractile so as to vary the capacity of the 

 buccal cavity , like the ( cheeks ' in most other mammals. As a 

 rule, in the present class, the mouth is terminal : when not so, a 

 rostral production, analogous to that in Sharks, makes the open- 

 ing inferior, as in the Tapir, fig. 155. In the Chrysochlore the 

 mouth is a small triradiate slit, like that of a leech, on the under 

 surface of the muzzle : it has a like inferior position, but is more 

 deeply cleft in Shrews, in which the groove that runs along the 

 mid-line of the under surface of the snout represents the third 

 ray of the closed mouth. The remoteness of the mouth from 



the end of the muzzle is in the ratio of 

 298 the length of the latter : consequently, 



among the Shrews, it is greater in 

 those (Petrodromus, Rhynchocyon, fig. 

 298) which, from the production of the 

 snout, have been called ( Elephant 

 HeadofRhynchocyon. lxxxiv. Mice': still more so in the Elephant 



itself, vol. ii. fig. 162. 

 The Ornithorhynchus subsists on aquatic insects, larvae, mol- 

 lusks, and other small invertebrates which conceal themselves in 

 the mud and banks of rivers, and is provided with a mouth 

 nearly resembling the flat and sensitive bill of a lamellirostral 

 bird. The jaw-bones are invested by a smooth coriaceous integu- 

 ment, vol. ii. fig. 199, A, e, a, devoid of hair, but perforated by 

 innumerable minute foramina. At the base of the jaws this in- 

 tegument-is produced into a free fold, which overlaps the hairy 

 covering of the cranium immediately behind it. The integument 

 covering the upper mandible extends beyond the margins of the 

 bone, and forms a tumid, smooth, and highly sensible border ; the 

 narrower and shorter under jaw is more closely invested : the 

 oral or upper surface of the lateral part of the under jaw supports 

 a series of about twenty nearly transverse folds, increasing in 

 breadth as they approach the angle of the jaw: the corresponding 

 surface of the upper jaw is smooth. On the outside of the pos- 

 terior part of each molar in the lower jaw, is the orifice of an 

 oblong cheek-pouch, fig. 3, F, f, about two inches in length, and 

 half an inch in diameter : the pouch is continued backward, and 

 is lined with a hard dry cuticle. The raised posterior lobe of the 

 tongue, fig. 212,/, with the projecting horny bodies,/?, g, can impede 

 the passage of unmasticated food to the pharynx, and direct it on 

 each side into the cheek-pouches ; whence the Ornithorhynchus 

 may transfer its store at leisure to the molar teeth, and complete 



