. Outlines of zoology. Zoology. PROTOTHERIA. 683 The vertebral centra have weak epiphyses in Orni- thorhynchus, and apparently none in Echidna. In the duck- mole the post-sacral vertebrae are stronger than the pre- sacral. The skull is smooth and polished as in Birds, for the sutures disappear. The rami of the lower jaw do not unite in front, have no ascending process, and have a slightly inflected angle. In Ornithorhynchus there are true mam- malian teeth, but only in the young; in Echidna none are present. Cervical ribs remain distinct for a time at least; the odontoid process of the second
. Outlines of zoology. Zoology. PROTOTHERIA. 683 The vertebral centra have weak epiphyses in Orni- thorhynchus, and apparently none in Echidna. In the duck- mole the post-sacral vertebrae are stronger than the pre- sacral. The skull is smooth and polished as in Birds, for the sutures disappear. The rami of the lower jaw do not unite in front, have no ascending process, and have a slightly inflected angle. In Ornithorhynchus there are true mam- malian teeth, but only in the young; in Echidna none are present. Cervical ribs remain distinct for a time at least; the odontoid process of the second vertebra is for a long time free from the centrum. Except on the atlas of Echidna, the cervical vertebrae are without zygapophyses or articular pro- cesses. The (meta-) coracoids reach the praesternum; there are also large epicoracoids and a T-shaped inter- clavicle, the whole girdle resembling that of Lizards. An interclavicle is, however, recognisable in the embryos of some Placentals also. In Orni- thorhynchus the ischia form a long FlG 29s. âpeMs of ventral symphysis; in Echidna the Echidna.â From Edin- acetabulum socket for the femur is bu.''gh Museum of incompletely ossified (reminding one "ence an rt- ,i t â¢, . 1 j S., Sacrum; Ep., epipubic of birds, though it is only a secondary tones; Ac, acetabulum; peculiarity); the pubes bear epipubic "/â .). of the tarsus, in the duckmole, there is a spur perforated by the duct of a gland. This spur is well developed in the males, but rudimentary in the females. The male Echidna has a similar but smaller spur. The fibula has a proximal process like an olecranon. The brain is smooth, the cerebellum is not covered by the cerebrum, there is a large anterior commissure, and the corpus callosum is rudimentary, or, according to Symington, absent. The food canal ends in a cloaca. The right auriculo-ventricular valve in Ornithorhynchus is partly muscular as in Birds, while in other Mammals it is membranous and worked by
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
