_Proechidna._[36]—The one known representative of the genus _Proechidna_ (Fig. 33) attains dimensions about equal to those of the largest race of _Echidna aculeata_. The skull is less depressed than in the latter, with the anterior portion of the palate very concave, and the deflected beak nearly twice the length of the remainder of the skull. As a rule, there are only three claws to each foot; but the first and fifth digits are represented by several phalanges, and one instance is known where there are five complete claws on the anterior and four on the posterior feet. There are two more vertebræ in the dorsal and lumbar region than in _Echidna_. The head and body are covered with a thick coat of hair, among which there are a number of short spines in the region of the back, which are much less numerous than in the typical race of the last species. The colour of the fur is generally dark brown or black, but the head may be almost white; and the spines are usually entirely white, although in certain cases they may be brown at the root. This species is known only from New Guinea, the recorded specimens being from the north-western regions of that country. It inhabits rocky ground, and dwells chiefly in the mountains, the specimens which were first described having been obtained at an elevation of about 3500 feet above the sea level. The Papuans capture it by digging trenches in the ground to a depth of about a yard, by which means they generally come upon its runs. _Fossil Species._—Remains of a species of Echidna of very much larger size than the existing forms have been obtained from the cave-deposits of New South Wales, which appear to be of Pleistocene age. This species was named _Echidna oweni_ by the late Mr. Krefft, but was subsequently called _E. ramsayi_ by Sir R. Owen. In referring this species to the genus _Echidna_, that term must be regarded as including _Proechidna_. CHAPTER VI THE SUBCLASS METATHERIA OR DIDELPHIA _General Characters._—The Metatheria or Didelphia are represented at present by numerous species, presenting great diversities of general appearance, structure, and habits, although all united by many essential anatomical and physiological characters, which, taken altogether, give them an intermediate position between the Prototheria and the Eutheria. Although the striking differences in external form, in many anatomical characters, and in mode of life of various animals of this section might lead to their division into groups equivalent to the orders of the Eutheria, it is more convenient on the whole to adhere to the usual custom of treating them all as forming one order called MARSUPIALIA,[37] the limits of which are therefore equivalent to that of the subclass. The more essentially distinctive characters are as follows.