many other mammals." In Echidna, too, but not in Ornithorhynchus, the hemispheres are well convoluted, though the arrangement of these convolutions cannot be brought into line with what is known concerning the convolutions upon the hemispheres of other mammals. It had been stated that in these animals, at least in Echidna, there were only two optic lobes, as in lower vertebrates, instead of the mammalian four. The late Sir W. H. Flower set this matter at rest,[1] and showed that Echidna was in this respect typically mammalian. The absence of the corpus callosum is one of the principal features separating the Monotremes from other mammals.
The Monotremata are represented to-day by two types, Ornithorhynchus and Echidna, which are no doubt worthy of being placed in separate families. Fossil remains of the group (apart from the problematical Multituberculata) are only known from Pleistocene times in Australia, and consist of the bones of a large species of Echidna, and some fragments of Ornithorhynchus, indicating a smaller animal than the living Platypus.
Fig. 53.—Brain of Echidna aculeata, dorsal view. (Nat. size.) (From Parker and Haswell's Zoology.)
Fam. 1. Echidnidae.—This family contains two genera, of which Echidna is the older and much the better known. The skin is abundantly covered with spines, with which are mingled hairs. The snout is tapering, the tail rudimentary, and the fingers and toes five in number. The spur and gland upon the calcaneum are smaller than in Ornithorhynchus. The claws are very strong, serving to tear open the ants' nests, upon the inhabitants of which the Echidna feeds, licking them up with a long extensile tongue like that of Myrmecophaga. In relation to this habit the salivary glands are enormously developed, and indeed the animal has been confounded with Myrmecophaga,[2] as the vernacular name "Australian Anteater" exemplifies.
