seventeen pairs of ribs and only two lumbar vertebrae. The skull is expanded in front, and the bill is supported by two, at first diverging, and then converging, premaxillae. Between them is the famous "dumb-bell shaped bone," which is believed to be the representative of the reptilian prevomer. The pterygoids are smaller than in Echidna, and the hard palate does not extend so far back as in that genus. The brain of this genus is smooth.
Fig. 55.—Duck-billed Platypus. Ornithorhynchus anatinus. × 1⁄6.
The discovery of the real teeth of Ornithorhynchus only dates from the year 1888, when they were found by Professor Poulton[[60]] in an embryo. Later Mr. Thomas found[[61]] that the teeth persist for a considerable portion of the animal's life, and are only shed, like milk teeth, "after being worn down by friction with food and sand." We have already (p. [98]) called attention to the general similarity of these teeth to those of certain of the earliest Mammalia and of mammal-like reptiles. The teeth are all molars, and they are either eight or ten in number. They are replaced by the horny plates of the adult animal; but the mode of replacement is curious. The plates are developed from the epithelium of the mouth, but round and under the true teeth; the epithelium of the mouth grows gradually under the calcified teeth, a method of growth which has possibly something to do with the shedding of the latter. The hollows and
grooves in the plates are the remains of the original alveoli of the teeth.
Fig. 56.—Skeleton of male Ornithorhynchus. Ventral view. The right fore-limb has been separated and turned round so as to bring into view the dorsal surface of the manus. The lower jaw is removed. acc.tars, Accessory tarsal bone supporting the spur; ant.pal.for, anterior palatine foramen; ast, astragalus; atl, atlas; ax, axis; bs.oc, basi-occipital; bs.sph, basi-sphenoid; calc, calcaneum; cbd, cuboid; cerv.rb, cervical rib; clac, clavicle; cond.for, foramen above inner condyle of humerus; cor, coracoid; cun, cuneiform of carpus; dent, horny dental plate; ect.cun, ecto-cuneiform; ent.cun, ento-cuneiform; ep.co, epicoracoid; epist, episternum; ep.pb, epipubis; fb, fibula; fem, femur; for.mag, foramen magnum; glen, glenoid cavity of shoulder-joint; glen, glenoid cavity for mandible; hum, humerus; in.cond, inner condyle of humerus; inf.orb.for, points to position of infra-orbital foramen; infr.proc, inferior processes of caudal vertebrae; int.rbs, intermediate ribs; isch, ischium; mag, magnum of carpus; max, maxilla; max.for, maxillary foramen; metat.I, first metatarsal; metat.V, fifth metatarsal; nas.cart, nasal cartilage; obt, obturator foramen; ol, olecranon; out.cond, outer condyle of humerus; pal, palatine; pat, patella; post.pal.for, posterior palatine foramen; pr.max, premaxilla; pr.st, presternum; pter, pterygoid; pub, pubis; rad, radius; scap, scapula; scaph, scaphoid of tarsus; scaph.lun, scapho-lunar; ses, sesamoid bones of wrist and ankle; sp, tarsal horny spur; sq, squamosal; tib, tibia; trd, trapezoid; trm, trapezium; tym.c, tympanic cavity; uln, ulna; unc, unciform; vom, vomer; x, dumb-bell shaped bone; zyg, zygomatic arch; I-V, digits of manus; V, foramen for fifth nerve. (From Parker's Zoology.)
The Duck-billed Platypus is, as every one knows, an aquatic animal. It is not found all over Australia, but is limited to the southern and eastern parts of that continent, and to Tasmania. The animal excavates a burrow for itself in the bank of the slow streams which it frequents. The burrow has one opening below the water and one above; and it is of some length, twenty to fifty feet. The Platypus feeds upon animal food, chiefly "grubs, worms, snails, and, most of all, mussels." These it stows away when captured into its capacious cheek-pouches. The food is then chewed and swallowed above the surface as the animal drifts slowly along. Dr. Semon, from whose work, In the Australian Bush, this account of the animal's habits is quoted, thinks that in the nature of the food of the creature the explanation of the loss of the teeth is to be found. He is of opinion that for cracking the hard shells of the mollusc Corbicula nepeanensis, upon which Ornithorhynchus mainly feeds, the horny plates are preferable to brittle teeth. Ornithorhynchus is apparently not eaten by the natives by reason of its ancient and fish-like smell. Besides, it is hard to catch on account of its diving capacities, which are aided by an acute sense of sight and of hearing. When the Duck-bill was first brought to this country it was believed to be a deliberate fraud, analogous to the mermaids produced by neatly stitching together the forepart of a monkey and the tail of a salmon.