OCR Text |
Show 1880.] ARRANGEMENT OF THE MAMMALIA. 653 7% The malleus i-3 relatively very large ; and the " processus gracilis," which is singularly long and strong, passes between the tympanic and the periotic bones to the pterygoid, with which it is firmly united. Thus the palato-pterygoid apparatus is directly connected bv a "suspensorium" with the'periotic, as in the Amphibia and Sauropsida. As in these, the representative of the incus is extremely small, and that of the stapes columelliform. 8. The coracoid is complete, distinct, and articulates with the sternum. 9. The hip-girdle is provided with large epipubes; and the iliac axis is inclined at a large angle to the sacral axis. 10. The corpus callosum is very small. 11. There appears to be no allantoic placenta, though, from the obvious remains of the ductus arteriosus and of the hypogastric arteries, there can be little doubt that the foetus has a large respiratory allantois. It is quite possible that, with a large umbilical sac, there may be an imperfect "umbilical" placentation. But, while the Ornithorhynchus and the Echidna are thus the representatives of the lowest stage of the evolution of the Mammalia, I conceive it to be equally unquestionable that, as Haeckel has already suggested, they are greatly modified forms of that stage- Echidna, on the whole, representing a greater, arid Ornithorhynchus a less, departure from the general type. The absence of true teeth in both genera is an obvious sign of extreme modification. The long tongue, extraordinary external auditory passages, and relatively large convoluted brain of Echidna, and the cheek-pouches and horny mouthplates of Ornithorhynchus, are other indications of the same kind. Hence the primary mammals, which were less modified, and the existence of which is necessarily postulated in the conception of the evolution of the group, cannot, without risk of confusion, be called Monotremata or Ornithodelphia, since in all probability they were as widely different from Ornithorhynchus and Echidna as the Insectivora are from the Edentata, or the Ungulata from Rhytina. Hence it will be convenient to have a distinct name, Prototheria, for the group which includes these, at present, hypothetical embodiments of that lowest stage of the mammalian type, of which the existing Monotremes are the only known representatives. A similar reasoning applies to the Marsupialia. In their essential and fundamental characters they occupy an intermediate position between the Prototheria and the higher mammals. 1. The mammary glands are provided with teats. 2. The cloaca is so greatly reduced that it is often said to have disappeared. Wirbelkorper," Zoologischer Anzeiger, 1879, No. 18), while admitting that Echidna has no epiphyses, describes epiphyses of an incomplete character between the posterior twelve caudal vertebrre of Ornithorhynchus. So far as I am. aware, the memoir, of which Dr. Albrecht has given a preliminary notice, has not yet been published; I content myself therefore with remarking that m y own "recent observations are in harmony with Dr. Albrecht's statement, |