The discovery of the real teeth of _Ornithorhynchus_ only dates from the year 1888, when they were found by Professor Poulton[62] in an embryo. Later Mr. Thomas found[63] 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 {114} grooves in the plates are the remains of the original alveoli of the teeth. [Illustration] 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_.) {115}